The idea here is really quite simple, but may be challenging to perform -- replace the face of George Washington on the scanned dollar bill below with someone elses and make it look as natural as possible. You can use the face of the playing card king provided here, use your own digital self-portrait, or something else (perhaps your dog or cat's face). Basic instructional hints below:

[I colorized and edited this scanned King of Spades just a little bit]

To begin as usual, save these jpg images of the scanned dollar bill and the replacement portrait to your Zip disk or FileShare folder (hold down the mouse button over the image, etc etc). Then open it in Photoshop at your workstation.

Starting hints:
You'll need to bring the king into a layer of it's own above the dollar.
Scaling and other transforms will be needed to get things correct in terms of position/proportion
Masking and other surgical techniques on selections to remove unwanted areas and backgrounds
Getting colors in agreement will be the hardest to do. Try different methods to solve this problem.

Below is an example of what a completed face transplant might look like:

Have fun! Good luck!