For the checkboxes, you can test the type of CC and act accordingly. The question is 'how do we know the checkbox has been completed' - is an empty checkbox a sign of incompletion or a negative answer? If you take a look at this thread you will be able to see how you can test for types of CCs or the CC title or tag properties -
https://www.msofficeforums.com/word-...it-macros.html
With the Picture CC, I would recommend you do some testing. I would try setting the CC to 'remove content control after editing'. If you have protected the document for forms, this will make the graphic uneditable since it won't be a CC.
I don't know that any protection you come up with is something I would fully trust - it will only stop the honest people. I would be pretty confident I could still extract your picture if you put it in a Word document with some form of protection that still allows me to see it.