Some people (designers, mostly) gave me crap for my post "tips for designers", but I have one more (and a few more for later):
DO NOT COMPRESS JPEGs AT 90%!!
I'm often left aghast by how people will start compressing their JPEGs to the maximum. I'd highly recommend using as little compression as possible. Sure, compress until the file size is acceptable, but please don't maximize the compression just because it will give an even lower file size! The compression ruins the image quality very much.
Use PNG. Especially if you're sending it to your designer.