Random Stills

This is an expression will generate a sequence of random cuts between layers. Arrange your graphic layers (movies, stills, etc.) at the top of the stack. Add a null layer at the bottom of the stack.

Add a slider control to the null layer.

Add the following expression to the slider control:

duration = .125; //length of each cut (seconds)
seed = Math.floor(time/duration) + 1; 

Add the following expression for opacity to each of the graphic layers:

slider = thisComp.layer("Null 1").effect("Slider Control").param("Slider")
if(index == slider)100 else 0

Adjust the duration parameter to suit your needs.