I've just developed the same issue in RH8. Many thanks for the common sense suggestion. It works for me. Hopefully, this issue will be resolved through an act of God shortly.
It sounds like the Insert Image dialog box has somehow fallen off the edge of your screen (Esc is closing the dialog box, returning focus to the main RoboHelp window). Try this:
1. Press Alt, Spacebar, M
2. Press any arrow key.
3. Waggle the mouse madly until the dialog box appears on your screen.
4. Click to drop the screen where you want it.
I was having the same issue and your suggestion fixed it. Thank you so much!!!
This solution works for Robohelp 7 and 8. Thanks so much.