Making a matte for your pictures – Method 2
This method will allow you to make a matte on a picture that is already in place, and it doesn't matter if it has been rotated or not. It also gives you square corners, not rounded ones, and does not clip any pixels from your picture. I used Photoshop CS3 in this tutorial.
Select the picture that you would like to use to create a matte.
Holding the ctrl key down, click on the thumbnail of the picture in the layers palette. This will give you marching ants around the edges of your picture. Under the Selection menu, choose transform selection. This will transform the selection only, NOT the picture. Right click on the W and H boxes up in the tool options bar to set the type of units you want to use for the height and width of your selection. I find pixels works the best for me.
Change both units to pixels, then add the number of pixels to the value in the W and H boxes based on the desired size of your matte. For a thin matte add 50 pixels to each box, use a larger number for thicker mattes. For example if the numbers in my W and H boxes were 400 and 600 pixels. I would change those to 450 and 650 for a thin matte. The area inside the marching ants will increase. Once you have the selection the size that you would like it, click on the checkmark.
Make a new layer and flood fill the selection w/ a color. At this point, you can use this layer for a clipping mask or to fill with white for white borders on your picture or whatever you would like. Be sure that your matte layer is beneath the picture layer, so you can still see the picture.
As you can see the picture now has an even border around the entire picture that has retained its sharp corners and you did not lose any pixels from the picture.