There is nothing built into Rh specifically for this purpose and Adobe do, of course, have access to rather a lot of developers. There is probably a javasript method on the web that would do this.
Another option could be to use Twisties if you are willing to compromise a little. What you could do is create a Twisty that contains the large image. Below the text of the Twisty (Click to see large image) you would put the small image. On opening the topic the user would see the text and the small image. On clicking the text they would see the large image with the small image below. I'm thinking that would not look too wrong and would be a whole lot easier for you.
See www.grainge.org for RoboHelp and Authoring tips
Thank you, Peter. I suspected Adobe might be doing something requiring more complicated development, but wanted to be certain. I appreciate the feedback and will investigate utilizing Twisties.
I created a hyperlink for the image, to itself, targeting a new window. This works fine in FF and Chrome which open the image by itself in a new tab. But IE prevents the link from working if the help system is on the local hard disk.
Yes, I can confirm that the mark is on, otherwise I get a yellow warning bar (security, activex, click to allow blocked content) each time the system loads into IE.