From: Pittsburgh, PA
The medium one is used (so far) for display on the front page when a member first joins. It will be used elsewhere in the future as we start expanding community-type stuff. So in your case, you don't necessarily need it now, but it will be good to have uploaded in the future!
One tech-related caveat: if you're changing up those images, please delete the one you want to replace before uploading a new one, or it will cause a known bug we're trying to squash in the next version of the upload tools.
Get out of the vacuum and express yourself!