Can police help me get an inappropriate picture of me as a minor off of my ex’s phone without legal backfire?

The problem you face here is if you still have those underage pictures on your electronics.  Even though the pictures are of you, it is still underage "kiddie porn."  Do NOT erase these images at this point.   You will want to discuss your situation with a lawyer in more detail. Choose someone from your locale.  Many lawyers offer a free phone consultation.  The lawyer should have a plan for getting you off the kiddie porn hook as they look to the Ex to remove the underage porn.   You may want to consider hiring a lawyer to send a Demand letter to your ex.  While a Demand letter has little legal impact, the lawyer's letterhead gets the recipient’s attention.   This is a relatively inexpensive way to put the person on notice that you are not going to let these actions continue and that you are serious about pursuing your legal remedies.  It also tells the person that they could get in legal hot water which will cost them a lot of legal fees if nothing else.  This will often convince them to stop their evil ways.