You cannot remove background images from the list in "Single image" mode. (You can remove them in "image list" mode)
1) Open "Desktop Preferences" -> Background
2) Select a background in the list
Expected: The "-" button will be enabled. Clicking the "-" button will remove the selected image from the list.
Actual: The "-" button remains disabled.
Not sure I care about this. Why does it matter? The contents of the list don't get saved between invocations anyway. (Which you might consider a bug, but it's kinda a big change and, again, I'm not sure how much I care.)
Ah. Yeah, I was just noticed that and was about to file that as a bug, too. If that is intended behavior, then I'll just close this. (For what it's worth, I think this is a confusing dialog; why go to all the trouble of have a list set up that people can add to if what they add will just go away as soon as they close the dialog? The 4.4 way of browsing to select a single image makes more sense to me).