When you take a screenshot, it takes a picture of your whole screen which includes everything.

If you use a Mac, there's other shortcuts you can use just to take a screenshot of the game by itself.

If you're using Windows XP you'll have to open up MS Paint and follow these steps:
1. Start -> All Programs -> Accessories -> Paint
2. Go to the Edit menu and click Paste or you can press and hold "Ctrl" and tap V.
3. Click on the selection tool and click and drag a box around the part of the picture you want.
4. Edit -> Copy, File -> New and then Edit -> Paste.
5. Click File -> Save As. In the box that pops up, change the "Save As Type" to PNG or JPG. Type in a name for your picture. Choose a place to save, like the Desktop, and click save.

If you're using Windows Vista, you can use the Snipping Tool which can be accessed through the Start Menu by typing "Snip" in the search box.