How do I add an image already in the page to a slide show ?

It seems so simple: I have a couple of images already spread out on a page, but later on I decide to collect them into a simple slide show after all. So I drag this Slide Show widget into place, and now ? Yeah, I know, I should use the button to select new images into it, or drag them as files from my Finder onto the Slide Show. But I already have them sitting on my page ! Isn't there a quicker and better way ? I tried copying and pasting in a lot of ways, and I can change the existing (three) sample images, but it's not even so easy to do it. Anyone ?

BTW, I know I could use Compositions, and at least they offer a simple + button to add extra instances to the widget. But I'd like to these plain vanilla Slide Show widgets in this situation.