I've had this issue for a while now, since I thought I could fix it myself. Almost all my programs have lost their icon image, which is not fixable by applying a different icon theme unfortunately. Just installed the Reversal icon pack to test that. My settings are attached here, sorry for the german:
I am assuming this is due to some of the weird behaviour I have had for some days months ago. Gnome would just not load and instead show me an error screen. So I had to uninstall gnome entirely, then reinstall it and that magically fixed it, but my icons were gone.
Wut?