Why do my Mac desktop icons keep losing their images?

Okay, here's a weird question: I have a Mac mini and a MacBook Air, both on the same iCloud account. The between iCloud and Dropbox the two are almost identical - a change on one shows up on the other. Except one thing...

I made aliases for a couple of folders, and keep them on my desktop, and replaced both folders with my own icons. (One is a .png, one is a .jpg, but I have also used .icns)

The problem is periodically the aliases will lose their images, which puts me in a loop. Change the icon image on one computer it won't change on the other one, and then if I change the other one the first one changes back to an image of a folder, and so on and so on, back and forth. Sometimes they will inexplicably sync, but then just as inexplicably revert to folders.

A very specific luxury problem. Any suggestions?

[Re-Titled by Moderator]