I realize this thread is a few years old, but I found it trying to solve a similar problem: I want to create create a matte for still photos I’m moving in the timeline. Now, if you use a “track matte”, it will track the motion of these photos. But here’s a cool trick–when you “nest” that layer of photos and then apply track matte, the matte will then remain static, while the images move within the matte. Just tested it–it works great!