App uses photo from Camera. But I want to use an already-saved pic.How can I do this?

There are some apps that when it requires a photograph, gives the user the option to open up Gallery, or some file explorer, or the Camera app (for you to take a picture right there and then).

There is one app that I use, however, that only has one option: Camera.

How can I somehow get the app to give me the option to choose a picture that I already have?