Free Image Editors
The following are free image editing programs which can be used to create graphics for wesnoth. These programs are Open-Source Software, like wesnoth, meaning they are free to use, and that you are free to look at the sourcecode.
For windows and linux, the program known as "The Gimp"; the "Gnu Image Manipulation Program" is recommended.
For the macintosh, this port of the the Gimp to a cocoa-based gui is recommended.
For the macintosh, Lemkesoft's GraphicConverter is an excellent program for preparing and compressing png images for the game, and may also be useful for the creation of images. The shareware fee is $30, although large parts of the program are fully functional for free.