Usually, placing the image in the header or footer is sufficient, but that can lead to issues with the image being replicated on
other pages.
An alternative is to:
1. Insert a continuous Section break immediately before the area that you want to remain editable. It doesn't even matter if there's
no paragraph mark before this, but you might want to format the section break's implied paragraph with 0 leading and a 1pt font
size - to minimise is visual impact.
2. Insert your image wherever you want it, and format its layout as 'behind text'.
3. With the image still selected, move its anchor so that its attached to the section break before the area that you want to remain
editable.
4. Protect the document for forms, leaving Section 2 unprotected.
The user can now type anywhere in the editable area, but cannot select or move the image. Two side-effects to be aware of are that:
a) the users can't change the first page header and
b) there are various editing restrictions with document protected for forms - these will be evident by the greyed-out toolbars and
menu items.