There is quite a bit of complexity to that image as it is. Someone spent a long time perfecting that and the effect is quite impressive. I aspire to make some of this quality some day, but for now I am learning. What makes many of these so difficult is camera movement. Those able to create these from scenes where the camera zooms or pans during a portion of the scene you want to leave animated boggle my mind.
This was my first attempt. Something simple.
This one required 2 masks.
This one required multiple masks, and I had to average two frames of background because the camera had a very slight zoom. I think you can still notice the door moving every so slightly, at least in the full sized version (FU Photobucket).
This one the Alien was swinging through other parts of the frame that I wanted moving, so there are multiple background images. The chains also didn't quite cooperate the way I would have liked. Getting things to line up so the the first and last frame are identical, so there is no "jump" in the animation is very difficult.
Three monitors and 6 instances of PS open are still not enough to speed making these up very much.