First of all, if you want to keep the size of your imag s consistent, make sure they are all the same resolution. Next, create a new document that is large enough to hold your images, and make sure it too is the same resolution.
Now just open each image and drag it on to your new "master" image document. Photoshop will place each picture on its own layer. Using the layer pallette you can activate any layer that you want and use the move tool to move the image to wherever you want to place it. You can also use the Editt/transform command to adjust the size of each individual photograph.