Although many graphics professionals turn to Windows or Mac OS to execute their designs, Linux is far from helpless in this area. While it helps that Adobe Photoshop, the undisputed gold-standard program that most professionals use for raster graphics, runs on Linux through Wine, there are several native Linux programs that offer some of the same functionality. Furthermore, there are many free vector graphics programs that can produce infinitely scalable graphics much like what Adobe Illustrator can do. Aside from the software situation, there is no reason why Linux could not be just as effective with graphics applications as OS X and Windows, since Linux supports many peripherals like tablets out of the box with full plug-and-play support.
Are the Linux programs drop-in replacements for Photoshop and Illustrator? The answer could be either yes and no, depending on the way you look at it and what your needs are. If you compare the Linux alternatives to Photoshop/Illustrator feature-by-feature, the free open source tools will come up short by a significant margin and there is simply no way to get around that fact. If you actually need those features on a day-to-day basis, then you should get your wallet out and purchase Photoshop and/or Illustrator. However, if you can get by with less, the free open source software tools may be enough to get the job done and save you considerable money in the process.
These tools are meant to work with raster (pixel-based) images. This type of image uses a matrix of dots of varying colors to create an image. Rasterized images have hard limits as to how much they can be manipulated in certain ways, but they support for more detail than vector-based artwork. Image formats like PNG, JPG, TIFF, PSD, etc. are raster-based.
The GIMP is arguably the most well-known open source graphics tool, and is one of the most mature open source image manipulation tools around. GIMP comes pre-installed in most distros, so there is seldom a need to manually install it. GIMP is quite powerful, with a large toolkit and support for layers/layer masking, channels, and over 150 filters to provide various effects. GIMP can handle all common image formats like BMP, PNG, JPEG, TIFF, etc. and can read many proprietary image formats like Photoshop native PSD and the various RAW formats used by some digital cameras.
GIMP is also very fast and has very good automation capabilities. Those familiar with Photoshop will find the tool window and filter organization to be to be fairly straightforward. GIMP includes the necessary tools for image cropping/retouching and works quite well as a lightweight (and we stress lightweight) Photoshop alternative. Unless you're a professional graphic designer, GIMP will probably be adequate for your needs.
However, GIMP does have problems. To begin with, its interface relies on a multiple window model that can be rather cumbersome for those not used to it. It is often essential to flag the tool window as “always on top” to keep it from hiding behind other windows whenever it loses focus. Fortunately, the GIMP developers are planning to convert the program to a single-document window interface in future versions; such functionality is already available in a derivative of GIMP called GIMPshop. Furthermore, GIMP has no CMYK support while other similar tools do.
Krita is a KDE application and part of the Koffice suite. Named after the Swedish word for “crayon”, Krita's layout is roughly similar to Photoshop, with a tool panel to the left and layer/color selection panels to the right. Although Krita is not quite as capable as the GIMP (owing in part to Krita's relative immaturity as a fairly new project) it can still do quite a bit.
When you first start Krita, you will be prompted with a preset template to use. There are several different sizes and color spaces (RGB, CMYK, and Grayscale) to choose from or you can define your own custom size and bind it to the desired color space. Once you have created or opened a document, you are able to use the toolkit panel (located to the left, just like Photoshop and GIMP) to work on it. Krita's layer and color management tools (and all the panels in general) are nicer than those in GIMP since they can be docked whereas GIMP's can only be free-floating.
Krita does have its deficiencies, but that is understandable since it is still fairly new compared to GIMP. Krita has about half of the filters that GIMP does, but the ones that are present work well.
Phatch is rather unique. It is not an “editor” in a true sense, but is more like an image processor. Phatch has many preset image effects that can be applied to an image; these effects can be stacked on top of each other and be applied in a specific sequence if desired.
Phatch offers batch processing capabilities; if you need to apply effects like sharpen, add border, etc. to many images at once, it would normally take hours if you had to do it manually. In contrast, Phatch can process entire directories at once in minutes. Alternatively, Phatch offers drag-and-drop processing. Phatch may not be a tool you use every day, but it is invaluable in the situations where you do need it.
Unlike rasterized images, vector graphics incorporate true lines, curves, etc. While rasterized images do not scale well since they are made of a finite quality of pixels (zooming in on a raster image will reveal jagged curves), Vector art may be resized or otherwise manipulated infinitely and still remain completely smooth. Linux has many programs that can create vector artwork.
Inkscape is in our opinion the best open source vector image editor that we have ever seen. We feel that Inkscape even gives proprietary tools a run for their money, based on the screenshots we have seen depicting what many talented artistic people have been able to do with Inkscape. Inkscape supports layers and has all the tools a program of its type should have, like freehand drawing, calligraphy brush, line/curve drawing, pen tool, path node management, circle/polygon drawing, gradients, color/stroke management, etc. All of these tools are logically placed and are easy to use. Inkscape also includes multiple color pallets and even some rudimentary flowcharting and 3d tools.
Additionally, Inkscape's native file format is Scalable Vector Graphics (SVG), so no further conversion is necessary. It can also import many file formats, both vector and raster. The only deficiencies that we found with Inkscape are the dialog windows that control fill and stroke; they could be more prominently placed than they are. Even if you just dabble in artwork, you should spend some time playing with Inkscape.
Xara is a formerly proprietary Windows vector drawing application that was later open sourced and ported to Linux. Unlike the other solutions mentioned here, Xara probably won't be found in standard distro repositories (although unofficial repositories may have it) and as such can be more difficult to install. The standard procedure to install the binary is to download the package file from the website, chmod it as executable, and then run it like a shell script. This will launch a graphical installation utility. We took a shortcut and installed the binary instead of compiling from source (which is what people are most likely to do) on Ubuntu 9.04. While the binary worked, it was very unstable and prone to crashing at random moments, especially when applying effects. Presumably, performance and stability would be better if the application were to be custom-built for each system.
Xara includes the standard tools and features of most vector graphics programs, but we noticed that the pen tool behaved differently than in other programs, which might put some people off. Xara included some nifty beveling, transparency, and contour tools in addition to the standard features, but the unstable nature of the program made us hesitant to experiment very much. (it was aggravating to repeatedly create something only to have the program crash without warning and take our work with it) Xara saves its native files in a proprietary format (.xara) but it can export to SVG and rasterized formats like PNG or JPEG. Like Inkscape, Xara can import many types of graphics. Xara shows promise, but for now, it would be best to stick with Inkscape unless you want to compile Xara yourself.
OpenOffice.org's Draw module is different from many of the other drawing tools we tested; while it can create simple line art in the right hands, it just doesn't have the flexibility of something like Inkscape or Xara. However, Draw appears to be a very promising tool for creating flowcharts as an alternative to Kivio (another flowchart utility). An added bonus to using it for this purpose is close integration with the rest of OpenOffice. We noticed that creating flowcharts with Draw is very straightforward and the various shapes fit the purpose nicely with pre-defined text areas.
Draw saves its native files in ODF Drawing format. While this is not a bad thing, it makes it difficult for other programs to work with it. While Draw can export SVG, it requires that a Java Runtime environment be installed. Draw's import functionality is adequate, but it can't handle as many formats as the alternatives we tested.