Well it sounds like someone stole your actual photos right? That's one thing. That is for sure plagiarism under any definition. But if someone painted one of your photos, you might not like it, and I get that, but it's not plagiarism or theft. If I paint a house did I plagiarize the architect? If I paint a car did I plagiarize the car designer? When you jump mediums like that you're creating a new and unique work that has value in and of itself.

Lots of artwork is inspired by other sources. Something being inspired by something is a lot different from theft.