GIMP (GNU Image Manipulation Program) is a cross-platform, freeware 2D graphics editor that is widely popular in the FreeSpace community. While not as sophisticated as the commercial Adobe Photoshop or Paint Shop Pro, GIMP is a decent enough utility for nonprofessional endeavors such as FreeSpace modding.
Like Photoshop, GIMP can work with layers, layer masks, alpha channels, adjust brightness/contrast values and there is a reasonable array of tools and filters that make it nearly as powerful as a commercial graphics editor.
GIMP can be used for all FreeSpace modding activities that require 2D graphics, such as making planets, shine maps, or missile trails. It can also be used for converting from one file format into the most advanced graphical file format, DDS, which is not supported natively, but a plugin is available for download.