If you knew how much time I've wasted on trying out the various wallpaper changers available for Gnome 3...
Bottom line. I think wallch is the best. First off it is in the repos. It doesn't have all the great options that wally has, but wally has one big drawback.
When wally changes the background image, it has a momentary "gap" where it shows only the system default wallpaper - and that gap can be up to 3 seconds.
That is just lame. So I went with the fewer options, but better experience of wallch.
Funny, because wallch was one of the first ones I tried. But the default options aren't great for gnome shell. I had to disable the "unity progress bar" and enable the "minimize to tray" options.
I would love to have all the options for online images that wally has, but that "gap" is killing me.
Now, if I could just find a changer that didn't cause the system to loose responsiveness for an instant when the change occurs...
EDIT - I'm also looking at Backslide which is a Gnome-Shell Extension (install easily from here)