If I am reading your needs correctly, I would extend the Duration of that last Still Image (click+drag on its Tail, and then add a Dip-to-Black Transition. Note: you can alter the Duration of Transitions in Edit>Preferences>General>Video Transitions (for your case - but you can also change the Duration for Audio Transitions too).
If you need a bit longer "black," then go to the New Icon in the Project Panel, and from the drop-down, choose Black Video. This will be created at the Duration for Still Images, that you have set in Edit>Preferences, but again, you can click+drag on its Tail, to change that Duration.
Is that helpful, or did I completely miss what you are trying to do?
Perfect! the new black video with a film dissolve is exactly what I was looking for. Thanks for the help.
Great. I was afraid that I was missing something very important. Glad that I guessed correctly.
Good luck, and happy editing,