From original image add a new layer and rect marquee tool to create border and add 25px white stroke. Then used a layer mask on the border layer to cut out the bits where the horse and rider are jumping out.
step 2 of 3
lots of time spent erasing and using pen tool and quick mask to cut away RH side of image around the horse and rider. Add new layer and fill with mid grey as background
step 3 of 3
Free transform to reduce the image a little and add a drop shadow to the horse layer