I don't think that is possible as, correct me if I am wrong, even within RoboHelp the images don't get updated automatically. It's something you have to do.
It's a good point though so I suggest submitting a feature request.
The more people who request a feature, the more likely it is to be actioned. Please follow this link.
See www.grainge.org for RoboHelp and Authoring tips
I'm guessing there isn't even a script that could be set up to sync the projects before compile? That's rather limiting, then.
I found a way around this!
Robohelp doesn't care whether the files are updated through the application or through Windows Explorer (such as by copying the most recent file to the project's folder).
I created a simple batch file that copies and replaces any file that is older than the file located in the resource manager. This allows Robohelp to pull in the updated files during a command line compile. There's always a way!
However, I found something problematic with the command line compile. I started a new thread to ask about that.