I don't think you can actually "crop" to a circle.

However, getting the result you want is very easy and quick.

1) CHANGE BACKGROUND to white. (In toolbox, you may have to click the black/white mini-icon to set the colors to black/white, and then you may have to click the double-arrow mini-icon to reverse foreground and background colors.)

2) SELECT a circle using the oval-shaped (it's really a circle in it's normal incarnation) selection tool located to the right of the rectangular selection tool.

3) INVERT the selection (in the menu: Select, Invert).

4) DELETE the selection (type the DEL/DELETE key on your keyboard). If the result is black surrounding your image area, then UNDO (ctrl-z) and start over at step 1 to get the background white instead of black. 
1) Click on the clone tool. It looks like a stamp.
2) Look down at the toolbar for the clone stamp. You will probably want to adjust the opacity. If it is less than 100, it will look more natural. Also, depending on what you are covering, adjust the scale. One last thing you can do is change the brush and the brush size.
3) Hold down the CTRL key on the keyboard while using your mouse to click on the area you would like to duplicate. You can change the size of the clone brush by moving the scale button.
4) After selecting the area you'd like to clone, let go of CTRL and click on the area you'd like to cover. You can click on as many areas as you like and the same cloned image will appear there.