Stop motion animation and audio keyframes in AE
You can choose which frame of footage is shown by enabling Time Remapping for the layer. Find it in the layer menu, or by right-clicking a layer in the timeline, or by using ctrl / ⌘alt / ⌥t



Turn it on for a layer and the displayed time is set by the value of the Time Remap property, which can be keyframed or controlled with expressions. Once you've applied it hit u to show it.



The value of the property is in in seconds, so you'll have to convert it to frames. You'll have to check what fps value the footage is interpreted as having.