First you need to call your image as follows for example:
This results in your image with a thin white line, creating the reveal you would get from a beveled mat cut:
Than you throw a span tag around it:
The 10 pixels of black make the frame and the 25 point padding makes the grey mat. The color of which is determined by the background tag.
The complete code for this would be:
As you can see, I usually put a