Here is one way to crop a picture in an odd shape like the one in the lower right corner of your example.
Activate the Rectangular Marquee tool and draw out the shape:
Copy the selection to the clipboard (Edit > Copy, or Ctrl-C).
File >New from Clipboard. This creates a new file with the shape on transparent background:
Save this file in a format that support transparency such as PSD, GIF, PNG.
Do the same with the other pieces of your collage.
You can then create the collage out of the separate pieces. If you don't know how to do that I will supply details.
Thanks for the help! I will try these methods out.