Ok now the fun part. Select the "Pen Tool", right click on the image and select "Fill". Pick White. Then right click again and select "Stroke" and pick "Brush". Then right click for the third and last time and select "Delete Path".

The pen tool and the paths are very powerfull in photoshop, as are the brush options. The stamp border is a great example of something that can be done very easily with those tools that would otherwise be rather difficult to do.

Anyway after you apply all of the above steps your image should look something like this:

Now put the pic you wish to use on the stamp. I am sure you will be able to do this by yourself but just in case I will describe my favourite way of doing this:

Open the "fav pic" you will be putting on the stamp. Reduce the size so it is near to the stamp's size.
Make a selection on the stamp that covers the area of the future pic and drag the selection out of the "stamp pic" to the "fav pic". Now select the "Move Tool" and drag the selection back on to the "stamp pic". This should create a new layer with the picture on the stamp.

This all sounds a bit tricky but I find it the best way to get the pic on to the stamp. There are other ways too and I am sure you will manage.