It's probably correct that this won't work in your situation.
The main alternative is to separate the form into sections by inserting section
breaks, and then protect only the sections that contain form fields. In the
Protect Document task pane, when the protection type is set to "Filling in
forms" and there are two or more sections in the document, there will be a
"Select sections" link to let you uncheck the sections that should be
unprotected.
That will let users insert pictures in the unprotected sections, as well as
entering and editing text. Some things won't work there; for example, nothing
involving the Drawing tools (AutoShapes, WordArt, etc.) will be enabled.