I think I might create a slideshow dvd, then import it into my dvd authoring program as a separate video from my main feature. This way I could create a menu with a selectable button to access the slideshow.
I use DVDLab Pro, which is rather expensive, but has a fully functional trial that is good for 30 days I believe. It can create the slideshow and, unlike GUIforDVDAuthor, it can accept an already authored dvd-video as input. The help file included with DVDLab Pro is one of the best I’ve seen, so look through it carefully on how to add a second movie to your first and how to create and link movies to the buttons on the menu.