The GNU Image Manipulation Program, or the GIMP, is a free, open-source image editor with much the same capabilities as commercial image editors. It runs on most popular operating systems.
Fixing transparency for IE6
One noteworthy feature is the ability to set the "background color" element in PNG images. Internet Explorer 6 does not handle PNG transparency, and will always show this color rather than what's actually below the image, so setting this color to the expected background color (#333333 for the default WoWWiki skin) will circumvent ugly border displays in most cases.
- Select #333333 as the background brush color.
- When saving the PNG, make sure to check "Save background color".
Reducing PNG file size
Using Tools → Colors → Posterize reduces the number of unique colors in the picture. Levels 25-30 reduces it to a few thousand colors without ruining pictures visually, and works wonders in PNG compression — expect files to at least halve in size and often more — at least for full-color pictures. Icons will likely not benefit a lot from it as they are fairly simple in the first place.