|Here's a filter that will do what you want:|
1. Size your photo however you like with motion/scale settings
2. Drop the Radial Shadow filter on the clip (perspective folder)
3. Set the filter thusly:
Shadow color as desired
Projection Dim: start with 7; adjust to make border width as desired
Softness: start with 0 or adjust as desired
Render: as desired
Shadow only unchecked
Resize Layer: CHECKED
That will provide a border color of your choice and width and it will adjust as you scale or move the image.
Tip: Set it how you want, then paste the same filter into the same clip again and change the 2nd one to a contrasting color. Now you will have a matted photo look. Add a subtle soft drop shadow to sell it a bit more (put the drop shadow after both radial filters).