Can you dynamically request a (cached &) scaled image via imagecache in css?
The URLs imagecache produces are pretty reliable, and contain the name of the preset. Looking at the code in imagecache_create_url() you could build up a path in your CSS that looks something like this:

/sites/default/files/imagecache/preset-name/path/to/image.jpg


Assuming that your files are stored at /sites/default/files