Right click the backround and press where it says " Change Desktop Backround". That should work just change it to another picture. If you want to put you own picture right click it and press " make as desktop backround". But change it first, i know all about macs so if this doesn't work comment and i'll try to help further.

obviously thats a stupid question

Of course you need God to exist, or whatever your religion calls him. Logically, think about it. Without a "God" , how would anything been made? There has to be a external power outside of this universe.