At least for what I have been wanting to use it, John's Background Switcher have been my to go program for years now. I wanted an app where it would use my handpicked pictures and after half a day it would switch them around to another. It is lightweight, sits hidden in the system tray and if you double click it's icon, it will replace the backgrounds, in case I wanted to flip through some of them manually. Also, if you have multiple monitors, it will place different backgrounds on each of them.
I am giving it 5 stars, but again, only for what I wanted to use it for, as I have never delved into it fetching pictures online from different sources.