Each higher numbered track overlays the tracks beneath it.
So, let's say you have your video on track one. Drag the still image to track two, above the track one section where you want it to be seen.
Then click the FX icon on the track-two timeline and apply the desired effect/s: crop, scale, zoom, etc...
Track one will become the background. If the still image overlay is the same size or bigger than the video on track one, the still alone will be visible.