[quote='Cogwheel on the Official WoW Forums',[url]http://forums.worldofwarcraft.com/thread.html?topicId=96143900&sid=1]If[/url] you would like to use custom icons for your macros, you can place them in your World of Warcraft\Interface\Icons folder (creating this folder if it doesn't exist). The files must follow the same guidelines for UI textures. Namely, they must be either BLP files or 24-bit/24-bit+alpha uncompressed TGA files. Their dimensions must be powers of two up to 512 (e.g. 32x32, 512x128). Note: any images that aren't square will look squished on your action bar.[/quote]

I use [url='http://www.gimp.org']gimp[/url] for most my image needs, but any program that can save tga's for you is fine. BLP I think is a blizzard proprietary format, and there are converters I believe but just doing TGA's is easier.

You can, of course, just make images with text on them or make one image with a suitable plain background to use and let your action bars show the text label of the macro. Every bar addon I've used will truncate all but very short macro labels though.