Size could definitely be a concern. I’ve got no idea what my requirements will be, but I at least know what the constraints are. For a baseline, each image can be up to 256KB. In theory, you can have 10,000 playlists (each with an image), but I wouldn’t actually want to have more than one or a handful at the most so that it doesn’t get in the way of the actual user’s playlists. Of course, there’s a good chance you wouldn’t get all 256KB. They may let you send in arbitrary data, but I wouldn’t be surprised to find them erase anything that’s not a valid jpg. Also, if they re-compress after upload, that would be another hurtle.

But, generally speaking, this is a way to allow folks to write arbitrary data to the net that you can use for syncing (or whatever, but syncing in my case) without having to run a database or user accounts. And, once you can do that, it’s game on.