Quick way:
Open you picture. Then use the Custom Shape Tool to draw the shape you choose over your picture. Your going to use this shape as a guide to cut out your picture, so move it around and shrink/enlarge until it's in the correct position over your picture. (You can decrease the opacity on the shape in the Layers Palette to help see through the shape).
Now, while holding down the Ctrl key, click on the shape layer in the Layers Palette. You should get marching ants around your shape. Next, and this is the key part, click on your PHOTO layer in the Layers Palette so it is the active layer.
Finally, since you don't want to cut out the inside of your shape but the outside, hit Ctrl + Shift + I which will select the inverse area. Now hit Delete. This should erase everything on your photo but what was inside the shape. However, you might not be able to see it because the shape is in the way. Delete the shape layer now.
HTH. PM me if that didn't make sense.
