The png library is being included, and lives in the same directory. This is an image from one of the links:
As you can see, libpng is in the folder.
And again, the only thing that changed was the SDL_image version to go from broken (on 2.0.2 and 2.0.3) to working (on 2.0.1).
And to clarify, this is a runtime error. Compilation succeeds without error.