Huge thanks to Pam from GirlyBits for figuring this out. She did 900% of the work.
this is a template to help you figure out how to make a banner that isn't terrible for your facebook group.
This is just a jpg, courtesy of Pam. Who knew she was a Facebook wizard in addition to making wonderful polish?
and here is a .png that I made
to save these, you should be able to right click and choose "save image". But I'm not a doctor, so your mileage may vary.