Synopsis: The GIMP (GNU Image Manipulation Program) is a powerful image editor for 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, an image format converter, etc. GIMP Portable is a portable package for the GIMP, so you can take your images with you and do your editing on the go.
Two add-ons for the program include a Photoshop-style layout and an unsupported "one window"-style setting. These are available from the website.
Alternately, another portable package is X-Gimp.