Create your form and put in the fields you require.
Immediately before you get to the part that needs the salesperson to insert a picture select Insert, then break, click the option for continuous.
Leave space for the graphic then select Insert, then break, click the option for continuous again.
Continue to finish your form.
Now select Tools and Protect document.
Notice on the right of the screen you get options of how to protect the document. The standard second option is Editing restrictions.
Place a tick in the box for Allow only this type of editing in the form and from the drop down select the option for Filling in forms.
Now click the blue writing under the drop down Select sections... and place ticks in only the boxes for the sections that are to be protected, in this example it would be sections 1 and 3 that would have ticks in them.
Remember to save your document as a template.
Bear in mind that the unlocked section will allow the user to add any amount of text/graphics they wish.
Hope this helps.