Ron

To clarify, BMP files are actually native the native Windows format, so they are always supported on Windows software. Applications more or less have to convert any import file format into a BMP in order to display it in a window. They are, as pointed out, not usually compressed (although there is a rarely used, or supported 8 bit loss free compression version). The latest version of the BMP file format (Windows XP, NT5 etc) supports alpha channels and jpeg encoding, but probably very few apps support this at the moment.

JPG files are probably the best format for distributing final images, due to their high compression, but they are lossy, so its a bad idea to save the original work in this format.

GIF and TIFF files both provide loss free compression, although the file size will be bigger that JPGs. Both are also proprietry, so support for them, now or in the future, is always questionable.

PNG also supports alpha channels and loss free compression, which is better than GIF, but maybe not as good as TIFF. It's non-proprietry so, although its fairly new, we should expect to see it become more widely supported in the future. Obviously Curious Labs jumped on the PNG bandwagon with ProPack, so it may be a good idea to consider this format for Poser exports.