If the circle is a separate layer
1) select all the pixels in the circle layer (ctrl+click on the thumbnail of that layer in your layers panel, you should see your mouse cursor change when you hold control and hover over the thumb)
2) select the layer with the photo
3) click the Add Layer Mask button at the bottom of the layers panel (looks like a rectangle with a circle in it)
http://i73.photobucket.com/albums/i215/estonc/grizzly_bear_chairs_2.jpg http://i.imgur.com/8kjZU.jpg