Instead of cutting a gaping hole through your window frame, you can combine two images by using the new Paste Special menu in CS5 (named Paste Into in previous versions). With this handy new menu item, you get to tell Photoshop exactly where to put the copied image. Your choices include:
Paste in Place. Use this command when you want to paste the image in the exact same position it lived within the document you copied it from. For example, if the image you copied was flush left in the original document, it'll be flush left when you paste it into the new document. The keyboard shortcut for this command is Shift-⌘-V (Shift+Ctrl+V on a PC).
Paste Into. Use this command when you want to paste an image inside the selection you've made (in other words, inside your marching ants). Photoshop puts the pasted image on its own layer and creates a layer mask for you, as Figure 7-2 illustrates. You see the pasted image only in the selected area; the layer mask hides the rest. Keyboard shortcut: Option-Shift-⌘-V (Alt+Shift+Ctrl+V on a PC).
Paste Outside. If you want Photoshop to paste the image outside your selection, choose this option. You'll get an automatic layer mask, although this time the area inside your selection is hidden; the pasted image shows only on the outside. Paste Outside is useful for swapping an image frame or border. (Apparently Adobe was running short on keyboard shortcuts because this command doesn't have one.)
If you're using an earlier version ...