Pretty much the default format I use for textures I'm uploading to
SL, is "TGA". Because the alpha channel rules are so well ingrained in my from my years of doing decals and textures for Mechwarrior, I save all my finished vendor art as tga, and then save off a smaller 370x370 image for use on my personal blog (so they images won't scroll on my webtab).
XstreetSL now allows upload of tga files, which it then converts into jpegs for use on the site.
It would be handy (and save me some work) if SLShopper allowed tga uploads as well. That way I could easily use my 512x512 vendor art without having to remember to also save a png of the full-size version.