GIMP

GIMP is a free, open source program for "photo retouching, image composition, and image authoring." Essentially, it is your all-in-one solution when it comes to anything that has to do with images. Its integrated scripting module allows GIMP to simplify what would usually take a few hours into just a few seconds. With native support for BMP, GIF, JPEG, MNG, PCX, PDF, PNG, PS, PSD, SVG, TIFF, TGA, and XPM image formats, the ability to handle image transparency, and the endless possibilities provided by user-made scripts and brushes, GIMP is the best image editor on the web.

Portable version available on PortableApps.com