Hi Ryan. How you do it can depend on how the slideshow was built and which version of Actionscript you used.
If your slideshow is timeline-based, then all you need to do it place the overlay on a layer above the slideshow layer(s).
If you used AS3 code to dynamically create the slideshow, and have the overlay as a movieclip, then it could be as simple as using just " addChild(overlay); " (where overlay is replaced with whatever instance name you assign it). That will place the overlay atop anything else on the stage. You might have to repeat it each time you add a new slide, though if the slideshow was contained within its own movieclip you would only need to do it once after the slideshow container is added to the stage..