I’m so sorry about the death of your boyfriend. 😳 It’s a shame that his parents can’t connect with you in a positive way, even now. But since you’ve tried, and failed, it’s time to just be frank with them. Write them a letter and explain that you are devastated over the loss of your boyfriend, but you are also very concerned that they have photos of you on his phone that were meant for his eyes only, and that it’s crucial that they’re returned to you and deleted without being copied so that no one else ever sees them because that would devastate you. Tell them you’d be very grateful for their cooperation, and request that they please confirm the photos are deleted. You have to say exactly what it is you want from them — you can’t play coy at this point. Hopefully, they’ll respond and do as you ask.
However, if you don’t hear from them, then you should consult an attorney because this is really about your privacy and making sure nude photos that belonged to him, don’t get blasted on the internet. There’s also some question over who owns the photos — and whether or not your boyfriend had a will, and if you were named in that will in any way. So, try and be graceful and direct with his parents, but if you don’t hear back from them, then I think you need to consult an attorney because if they won’t cooperate, this becomes a legal relationship — not just your strained relationship with the parents of your deceased boyfriend.
I hope that helps.