Image resizing is trivially accomplished through CSS using the real image tag:
[image: <url>|width: 200px; height: 200px]
Right now, the only way to get a post background is by enabling profile backgrounds on your user page; go to where you'd normally edit it, scroll to the bottom, and click 'more' next to 'submit'. (I know it's janko-grade janky, but it's still sort of a beta feature.) The only profile background you can have is a stretched and blurred version of your avatar. If you haven't changed your avatar at all in the last three or four months, you'll need to re-upload it, ideally at a larger size.