You may also wish to investigate the possibility of a restraining order against this person, so as to stop him from contacting either you or your sister. This is in addition to claims you have to get your property back.
Best of luck to you.
Clearly do it yourself is not working. You will need a lawyer for a restraining order and perhaps as interface with the police for their intervention.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
You have several causes of action in various fields of law. I have been handling such matters for years. You need at least one attorney to resolve the harassment, bullying and stalking issue to you and your sister. This is a serious issue and needs immediate attention because of the possible risks. Then you need to resolve the photo issue(s). Many reputable attorneys offer a free consultation. I suggest meeting with at least one soon, very soon.
The response above is not intended as legal advice since it’s impracticable to provide thorough, accurate advice based upon the query without additional details. It is highly recommended that one should seek advice from a criminal defense attorney licensed in your jurisdiction by setting up a confidential meeting. Moreover, this response does not constitute the creation of an attorney-client relationship since this message is not a confidential communication because it was posted on a public website, thereby publicly disclosing the information, which is another reason to setup a confidential meeting with an attorney.
Welcome to the world of working without a written contract. Enforcing verbal contracts are difficult as the result comes down to your word versus his word. If you have email communications between you two defining the agreement, that would be very helpful. You have copyright claims to these images that are likely shared with him. The important part is defining the grounds for exploiting the materials.
The harassment and stalking is another issue. You can always call the police if it gets threatening.
I any event, hire a lawyer to help guide you though this maze.
Contiguglia / Fazzone, P.C. / Standing up for you, when others sit down! / www.contifazz.com / Just because I answered your question, that does not create an attorney - client relationship between us unless and until there is a written fee agreement in place formalizing our attorney - client relationship.