Ard, first go to file then open as layers, then choose your picture that you want, then go to open as layers again and then choose your second pic and then ta da. Two pics merged together. If you want a particular picture to be on top, just go to the layer of the picture that you want to move up and go to 'layer' then 'stack' and then finally 'move layer up'. I was going to include screenshots untill I realised it was an incredibly easy thing to do. I hadn't used it before so I really just guessed and it worked. If you need anything else, just say.
