Since the last UCP update, there are 4 background images in total. One of the suggestions we received was to be able to choose your desired background image for some period of time (possibly a month) instead of it being chosen randomly every time a new page (re-)loads.
Background image 1:
Background image 2:
Background image 3:
Background image 4:
Background image 1:
Background image 2:
Background image 3:
Background image 4: