So... it appears that this is the place to put wallpaper. I was playing around with a photo I took, and tried putting the Ubuntu MATE logo on it. It turned out pretty well. It's higher than 4K resolution, so it should be good for just about any screen, even the new iMac Pro.
Anyway, here it is. If anyone likes it, let me know.
Edit: it doesn't appear to be uploading. Here is a Google Drive link: Picture