Displacement Map for Perspective

Greetings All,
I’ve tried in vein to understand displacement maps; I can find nothing on “The Internet” that teaches me what I need to learn. Now I’m looking to all you smart-types to solve my problem for me!
All I want to is take an image (a rectangle) and make it look like it’s going off into the distance (trapezoid-like), getting smaller from left to right.
This doesn’t even need to animate, I just need two states: straight-on and trapezoid-like – think iTunes album art browser minus the bells & whistles.
I know this can be accomplished with a displacement map, but I know not how to get 'er done.
Thanks in advance for your boundless expertise!

steve