Open the image of the mat in PSP. Open the image you want to place on it, drag the image layer form the layers pallet on to the mat image or copy paste it onto the mat image as a new layer.
Now with the Pick or deform tool (name dependent on Version) you can re scale by moving the corner nodes in or out you can change the mode to deform in perspective mode or shear or holding both shift and ctrl keys down you can have free form. Changing the image shape to fit another does need practise if it is to look correct (unless its a simple form) but an art worth learning. You often need to alter perspective on two planes