There are lots of free lossless conversion GUIs: FLAC Frontend, TLH, mkwACT, xACT, Monkey's, foobar

And you'll get the same results converting .aiff directly to FLAC with foobar2000 as you would converting .aiff to .wav with some program > FLAC with FFE if you use the same version of FLAC and the same encoding level.

It needs to be FLAC to reduce the size, so that less data has to be uploaded and downloaded. Once it's in FLAC, it will tend to be converted to .wav on a Windows machine, and .aiff on a Mac--if it gets converted at all.