texture import function in a finished project?

Hello everyone, I’m looking for a moment without any answer unfortunately, how to import a texture when the project is Package, I explain myself for a future project I would like the user (player) can import himself a texture use as heightmap to generate the landscape, unfortunately I can not find any solution for the moment. if anyone has an idea to realize it (create a file import function in a finished project so that the player can import himself a file type?
thank you in advance for your answer.