So I whipped up a script to do the job instead. Just replace the source path with the full path to your folder containing the wallpapers you want to set (it's near the bottom).
src = '/home/ubuntu/Pictures'
imgfiles = glob(os.path.join(src, '*'))
wp = [img for img in imgfiles if is_image(img)]
Then run the script from terminal.
~$ python wp_set.py
This doesn't work on older Gnome 2 desktops though. The script is available for download from MediaFire, for anybody interested.