You may have to add them to your baggage files?
Certainly it won't hurt anything to add the images to baggage files, but I'd be shocked to find that solves the problem.
If an image has been used in RoboHelp, it should be present in the project. End of story. The fact they seem to be disappearing seems to suggest that maybe something has gone awry with folder naming or image naming or the project in general.
So what version and flavor of RoboHelp are you using?
RoboHelp 10, RoboHelp 9, RoboHelp 8, etc.
RoboHelp HTML, RoboHelp for Word
That's what I thought - it was worth a shot ;>)
Perhaps this thread can help. It's only a workaround, not really a solution.
Short story is try renaming a folder to force RH to update its database of images.
We found this had to be done each time the CPD file was deleted (which we did each time an author worked on a project, to work around other issues.)