Simply save the image you wish to use as a layer, and open it in photoshop.  Then simply use the select tools (on your left, you probably want the one for drawing boxes, or you can use the magic wand or polygon lasso tools to select a non-square section of your image) then go to "edit" then"copy".  Then click the window with the picture you want to layer on to and click "paste".  You can use the tools on your right to change the opactiy and fill of the image, so you can see the background image through. 

Hope this helps!

Laura

http://www.douglasdrawings.co.uk