The "cheeky squirrel," originally posted at National Geographic and made famous by Neatorama's own Upcoming Queue, has now gone viral on the internet.
Hundreds of photos, including many famous ones, have been modified by adding the ground squirrel. Now you can do so with your own photo or one of your favorites from the web.
The simplest generator I've encountered is "The Squirrelizer." You cannot, however, upload an image from your computer; you will need to
use one that has a url (stored somewhere on a blog or in an
image library such as Imageshack).
After that, the generator is intuitive. If you don't have an online account where you can store the image, just take a screen shot of your creation.