I started looking at your contrib, and im wondering about something. It seems to me that everything is uploaded to your design directory. Wouldnt it make more sence to upload it to the "var" directory?
If i upload it in the admininterface now, the images are not available in the user site, because i use a different design.
Well, by the time i wrote it, it seemed to me that everything is stored qiuet random in the var directory. Also i liked the idea that everything belonging to the design is in one place.
But this decision is only suitable if you just maintain one siteaccess at a time, else it gets annoying.
If i would write the extension today i also would use the var directory for storage. This would also save me some other trouble in the code. One gets smarter everyday ;)
Perhaps i will take a day and complete my current additions to it, and make it use the var dir.