I’m not too sure how Gnome would work (I suspect
gconftool -s would be involved), but with Openbox and feh, it’s a very simple trick.
You could set this up as a script to run, but it should work in your
.xinitrc file as well.
feh --bg-scale potd-1600x1200.jpg
If I understand it correctly, the file name is always
potd-1600x1200.jpg, so you’ll probably overwrite the file each time it’s wgetted. (There’s a
--no-clobber option for wget, if I recall correctly.) If you see one you want to keep, you should probably copy it to a new file and give it a proper name.