Login screen mod

I was wondering if some1 could explain to me (rlly in depth cuz im blonde) how to changs my login screen. I have the picture i want i just dnt no wat to do wit it. Right now its saved in my pictures from paint. Do i have it make it a TGA file? Help plz! ill love u forever (maybe).

Or if some1 wants to get it ready for me i can give u the pic, but im ok wit doing it myself if i could just figure out how.