Are you able to post a copy of the form here for us to look at?
I seem to recall I had a similar problem, but I can't remember exactly what caused it, just that I did resolve it.
The approach I took was to rebuild the form bit by bit starting with the background formatting then adding text, fields, other controls, sub-forms and finally images in that order, checking after "each" addition to see if the problem manifested itself.
If you do this you may be able to identify the offending item and can then focus your efforts on the culprit.
Worth a try at least?
With images you might also want to look at layering i.e. where they reside in terms of sent to back - brought to front. Just a thought.