Sorry for sounding stupid but I am stuck at point 4.

1. Go to Developer>Restrict Editing
2. Click the 'Editing Restrictions' checkbox
(I have editing restrictions set to 'filling in forms')
3. Select the content control
I select the picture content control
4. Click the 'Exceptions' checkbox
Cant see the 'exceptions' checkbox
5. Click 'Yes, Start Enforcing Protection'
6. Add any passwords you require and click OK.