Hi there
Sorry, but RoboHelp does not offer a method to achieve this. Unfortunately that's a huge hint that Adobe isn't using RoboHelp to develop those pages.
If that's the effect you are after, you will need to roll your own with some JavaScript that does a hide and show on text and images.
One way you might achieve it with a single image would be to insert the image *TWICE* on the page. Then scale one pretty small. Take the full sized image and tuck it inside a Dynamic HTML drop-down. The net effect would be that the user would click to see the full sized image (revealing the DHTML drop-down) and the smaller version would be "pushed" farther down the page.
Cheers... Rick 