Portable GIMP is the popular the GIMP image editor packaged as a portable application, so you can take your images with you and do your editing on the go.
The GIMP (GNU Image Manipulation Program) is a freely distributed program for such tasks as photo retouching, image composition and image authoring. It has many capabilities. 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, an image format converter, etc.
GIMP is expandable and extensible. It is designed to be augmented with plug-ins and extensions to do just about anything. The advanced scripting interface allows everything from the simplest task to the most complex image manipulation procedures to be easily scripted.
To have a better idea of what GIMP can do, or rather what you can do with GIMP, browse through the tutorials.
Note: If you would prefer to have the installable version of GIMP, you can download it here.