Amongst the widgets that come with Mac OS X is the Tile Game. You shift around the tiles to reveal a picture of a snow leopard (or a leopard or a tiger, depending on your version of Mac OS X). If you play the Tile Game often, you might find the image a bit boring.
Luckily, changing the image is easy. Start by finding an image in iPhoto or the Finder to replace it. To avoid stretching, go for an image that is roughly square. Now click on the image and hold. While holding down the mouse button, press F12, or whatever key you have assigned to load up Dashboard. Now simply drag and drop your image onto the Tile Game.
Annoyingly, the image will change back to the tiger if you close the widget. Permanently changing the image is a bit more complicated. It essentially involves replacing an image file hidden within the widget file. If you are familiar with delving into package files, this is quite straightforward.
To change this image, first we need to find the Tile Game widget. It is located in the Library folder inside Macintosh HD. Make sure you don't accidentally go to the Library inside your user folder. Inside the Library, look for the folder called Widgets. In here you'll find the Tile Game. To get inside the package, right-click (or Control-click) on the file, and choose “Show Package Contents” from the menu that appears.
The file we want to change is called game.png, and is located inside the Images folder. Just replace this image with your new one, making sure you give it exactly the same file name.