you could also open a new document with a white background and then open your pic and drag it on to the background layer. Make sure the image size of the pic is the exact same as your background. Then once it is dragged over centre it so that you can't see the background and hold the ctrl button and E to merge the layers. Go up to Image on the the menu bar and then go to canvas size. In the canvas size window enter the dimensions of the matte you want.....if you want 1/2" all around your photo enter .5 in both fields and hit ok. This will give you a perfect matte every time and you don't have to fiddle with it to make sure your pic is centred on the matte. Then you can drag the matted pic onto you LO. I hope that helped too.....the others have great ideas too. Oh and if you want to change the colour of the matte just use the bucket tool and your colour pallet