It's been a couple months since this thread was updated, but out of hundreds of semi-related Multisite issues, this is the only one I can find that is similar to mine.
I have multisite 3.5.1. It's brand new. I can add new sites and map the domains ok now (took me about a straight 10 hours to figure it out but finally got it).
Images upload and work fine in posts and pages. But I have ONE subdomain that will not display the header image. It's just this one site. The image is uploaded and renders just fine in a browser by itself. The path to the image is published fully in style.css. All other images seem to work except this one image.
The site is here: http://rworldproperties.com/
I'm happy to share whatever details may help. Any insight is much appreciated!