First of all i would like to say: Great application you got there! I've got a few suggestions for improvement though. Personally i use wallbase alot myself to find new wallpapers and i have been searching for an application like this for a while.
My suggestions are:
Make it possible to not only download by search string, but also download newest or download toplist and something like that.
Also instead of making the program handle the background, then make it download to a specific folder so i can set windows to handle the background switching instead of constantly having this running in the background :)
At last i think it would be a good idea to talk to Yotoon (creator of wallbase) about the downloading of wallpapers from wallbase. I would certainly not see this program excluding wallbase, but i know wallbase has problems with bandwith traffic costs so
it would probably be a good idea to limit the download interval from wallbase.