It seems like every day I discover something cool that’s (probably) been around forever, and I just didn’t bother looking. So please humor me while I mention art.ubuntu.com.
I’ll let you look it over and get a feel for it. It works a lot like other galleries, although I like the random image effect and the way the thumbnails are individually managed. It makes it easier to handle them in bulk. And it looks like you get around 50Mb of space to work with.
I’ve already uploaded a few homemade wallpapers, plus the default Ubuntu wallpapers I’ve been collecting since Breezy. There’s probably not anything new or earth-shattering there, but you’re welcome to look.