Do you mean the desktop, or the login screen? The login screen cannot be changed. The desktop background can be changed under System Preferences > Desktop & Screensaver. Or, if you have a photo you want to use, right click it and choose Set As Desktop Background.
1. I bumped the keyboard and fixed it. Where is that in the manual!
2. Mathamaitcs is Mathematics on angry steroids! I actually took more History than math so that could be the issue. Actually it was ancient history (Greek and Roman stuff). I think they might of spelled it like I did.