Sure you can. Just open one of them, then go under the Image menu to "canvas size, and type in a bigger number. Make sure your "mode" is set to RGB (also under the Image menu). Open your other images, select all, copy, then go to the first image and say "paste." Each image will be in its own layer, (see the "Windows" menu, show layers) you can them move them around, and when you have them all in, say SAVE!