Is it possible to add an mp4 movie as a background vs just a jpg image? I like to have a background video that I green screen lego mini figs into.
Yes, you can:
Next you need to setup the chroma keyer below the background browser. Please be aware that the chroma key is applied to each frame individually and the result can't be changed once the frame was inserted into the movie.
Finally you can adjust the chroma keyer in the action gear menu item "Advanced Settings" which combines all the options above in one separate window.
Feel free to download a demo version of iStopMotion to test this on your own.
Does this work for you?