The newest Acorn release will include a JPEG compression/quality slider when saving, as well as a number of bug fixes (everyone loves those, right?). If you want to try it out now, you can hit the dev build, but the rest of us will be happy waiting for an official release I'm sure.
Mueller has also created a wiki just for Acorn, and it's got a few nice tips and tricks, a plugin section, and a writeup on the Acorn file format. It's pretty barebones at the moment. but hey, it's a wiki, and it just started. Give it some time (or some contributions of your own), and it'll likely soon be an invaluable guide to all the Acorn users out there.