I've looked pretty hard already for an answer to this, but I'll go ahead and ask it.
I have a sequence of .png files that I want to be able to switch through using an expression slider. Specifically, let's say it was a .png sequence of two frames, an eye open and and eye closed. If I wanted to determine when the eye blinks using an expression slider to hold a specific frame of the footage, what should I do? I need to use this method only, so please do not suggest other methods unless they allow me to hold frames of footage using keyframes.