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.
Jpeg files have been the bane of the micro-hero community since it's earliest days. The main reason being jpeg's quality to file size ratio. Jpegs are very versatile since the quality can be adjusted to fit file size restrictions. Unfortunately to get the smaller file size, you must set quality so that the colors bleed. It is very much possible to have high quality jpeg micros, but the file sizes end up being larger than bitmap format. And like bitmap format, jpeg does not allow for transparencies. The lack of transparency is the main modern reason to not use jpeg format for micros.
PNG files are larger than GIF files, but promote adaptive transparency and a way larger color pallet. But they do not allow for animations.
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.