So far there have been some errors in this thread about the minimap pictures.
The truth is that both minimaps are best at 256x256. I don't know if the other resolutions work or not, but the most optimized and stable version is 256x256. Most of the time, you don't have to worry about making an alpha channel; in most programs, an alpha channel on .tga's is just automatic. Also, the CORRECT, EXACT names for the two minimaps are:
war3mapPreview.tga - The preview you get when you host the game on Battle.net (should be a targa, or .tga).
war3mapMap.blp - The actual in-game minimap. If it's supposed to actually be used as a minimap, know that it's exceedingly tricky to align with the terrain correctly, but looks insanely awesome if you get it right (should be a blizzard picture file, or .blp).