GIMP is the GNU Image Manipulation Program. It is a freely distributed piece of software suitable for such tasks as photo retouching, image composition and image authoring. It can be used as a simple paint program, an expert quality photo retouching program, an online batch processing system, a mass production image renderer, a image format converter, etc.
SIP provides image processing, pattern recognition, and computer vision routines for SciLab, a Matlab-like matrix-oriented programming environment. SIP is able to read/write images in almost 90 major formats, including JPEG, PNG, BMP, GIF, FITS, and TIFF. It includes routines for filtering, segmentation, edge detection, morphology, curvature, fractal dimension, distance transforms, multiscale skeletons, and more.
XPaint is an easy-to-use image editor which supports many standard and less standard paint procedures that demanding non-professional users would expect. It includes several advanced features like image processing algorithms, programmable C scripts, simultaneous editing of multiple images, and supports a wide variety of image formats.
Tux Paint is a simple and entertaining drawing program geared towards young children. It has a simple interface, sound effects, and a cartoon character (Tux, the Linux penguin). Along with drawing brush strokes, lines and shapes, you can also enter text and place "rubber stamp" (or "sticker") images on the picture. Tux Paint is extensible, and could be useful in an educational environment (such as a grammar, elementary, or grade school). It's portable across numerous platforms, and runs well even on slower systems like the Pentium 133MHz.
Allegro Sprite Editor (ASE) is a program specially designed with facilities to create animated sprites that can be used in some video game. It lets you create static images, characters with movement, textures, patterns, backgrounds, logos, color palettes, etc. It focuses on pixel editing to do pixel art. It isn't a photo retouching tool or a vector graphics editor. Mainly, it is a tool to create tiny animations pixel-by-pixel.
F-Spot is a personal photo management application for the GNOME desktop. It simplifies digital photography by providing intuitive tools to help you share, touch-up, find, and organize your images. These tools include editing, color adjustments, tags, Photo CD creation, and exporting for upload.
GridFlow is a plugin for PureData, Ruby, and jMax. It provides the ability to process images and video as N-dimensional arrays ("Grids"). It consists of a framework for grid transmissions, elementary grid processing objects, picture-oriented processing objects, and input/output objects. It also allows one to write plugins for PureData and jMax using the Ruby language.
CinePaint is a deep paint image retouching tool that supports higher color fidelity than ordinary painting tools. CinePaint is used to retouch feature films and for professional photography. CinePaint opens high fidelity image file formats such as DPX, 16-bit TIFF, and OpenEXR, and conventional formats like JPEG and PNG. It has a flipbook for movie playback of image sequences in RAM. It supports 8-bit, 16-bit, and 32-bit color channels, HDR, and CMS.