On marketplace-feed-dev.allizom.org, we have a 1000x1000 500KB background image of a pug. We should contain the size to what we need (width ~= 300-320px), and optimize the image.
We should reduce the image to whatever size it takes to ensure that it is at least 320px wide and 150px high without affecting the aspect ratio. JPEG@80 is probably appropriate.
Added Aviary so that the image size is constrained to 320x240.
STR: Try out the new Aviary background image uploader.