How to make a custom shape crop?

Question

Images

Good afternoon! I do bling . And there is a section where people can send their picture, from which you can make a bling. But the picture is rectangular for a photo, you only need to take an area from it. I would like to indicate this area with a cursor, a finger on the mobile screen. I don't want the usual crop of a rectangular area. I want my curved stroke. Like in the example in the attached file.