Hi
The fade out of a final image or final clip should be a Cross Fade transition applied to the end of the timeline. This transition reaches full black at the join between clips i.e. at the end of the first clip when the next clip would normally fade in. As there is no following clip/image on the timeline the transition ends in black at the end of the final clip. Note that it will have the set duration.
You could of course, always use a transparency effect applied to a black blank clip place on the overlay track to achieve a similar thing but a Cross Fade transition is simpler. 🙂
Nat