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[]

How to set the background color in GIMP.
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".
- Done.
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.
External links[]
See also: PNG, JPG, WoWWiki:Image guidelines