http://www.qualibyte.com/pixelformer/

the best program to convert alpha pngs to alpha bmps. just make sure to select premultiplied alpha bmp when exporting

Another powerful editor IMHO is Greenfish Icon Editor Pro. It can also easily convert alpha PNGs to alpha BMPs, convert PNGs, BMPs, GIFs with no alpha transparency into such with alpha transpareny saving them to one of the above formats + ICO . As a matter of course you can also resize (stretch/crop) your images and some nice filters are also included.
GFIE Pro is freeware, requires no installation and leaves no traces in your Windows registry.

magic pink 2 alpha: