Eileen, this is very similar to a situation my team faced a few weeks ago, though our error was related to deleted topics and not images.
I'm not sure why exactly RoboSource throws up this error, but we did notice that the root.fpj file retains references to the deleted files, which makes RoboSource look for them, and when it doesn't find them it starts acting all wonky. Adding the files back doesn't work b'se for some reason RoboSource recognizes the deleted files as still being part of the project. Talk about ghosts in the system!
However, we did find a workaround that worked in our case. Open the root.fpj file in a text editor like TextPad, and remove *all* references to the deleted files. (In case of our topic, this also included the HHC file.)
Do post back if this solution worked for you, or if you found another fix for this issue!
Actually, we were using RH 8 with 802 applied when this error cropped up. We've moved on to RH9 and a whole different set of problems since.
Just to clarify, my topics didn't really go missing - I deleted a couple of troublesome topics from RSC (instead of being good and deleting from within RH as I should have) and then tried recreating them using RH, but I simply could not create them b'se RSC detected them as "already exists". When I tried retrieving these supposedly extant files, I couldn't retrieve them either b'se RSC prclaimed them "invalid".
Removing references from root.fpj worked for us - I think it made sure RSC wouldn't expect those files in the project.
P.S. - Your site is one of my first stops when I run into any RH-related issue. Thanks for all your help!