Micro-Heroes are mainly saved as the BMP, GIF, JPEG and PNG file types.
The basic most easiest way to create micro heroes is in MS Paint creating pixel by pixel bitmaps or BMPs. The earliest micros were distributed over internet mailing lists. As such, the file size of bitmap files proved too large for the mailing list's file size limits. With micros now being stored on image sever accounts and distributed on message boards, file size is less of an issue, especially with the advent of macro sized micros. Unfortunatly, bitmap files do not allow for transparency so most creators prefer not to use it.
GIF is the format of choice for micro making. Some creators complain about its lack of colors, but this is only a minor issue. GIF format keeps good color quality at very low file size. More importantly, GIFs allow for transparencies and animations. This is why GIFS are still preferred despite file size no longer being a major issue.
PNG transparencies don't always work on certain web browsers or message boards. Also the program you save the PNG with can cause transparencies not to work.