Hi Shorty, welcome.
I am wondering what exactly you mean but it sounds like basic Photoshop. But you are making it much harder. If I get you that is.
You are creating a blank document the size you want right? Then you are copying and pasting an image onto the document. When you do that, you make a new layer. The image on the new layer can be moved independently anywhere you want it by using the move tool, the one at the very top of horizontal toolbar, or press "v" key. If the image isn't the right size, you can transform it, via the edit menu.
Here's where you confuse me: All I know to do right now is to paste the picture onto the canvas, then draw a rectangle around it, cut it out, then set another rectangle where I want the new picture based, and paste it in there.
Why all these steps? It almost sounds like you're doing this on paper :mrgreen:
You don't need all these rectangles. That's the beauty of Photoshop. Everything can be manipulated separately because of the layers.
I hope this helps you but let us know if not. You may want to start a new thread addressing your specific problem if you haven't gotten it figured out.