def path_is_valid(path: Path) -> bool:
    if not path.exists():
        return False
    return True

There’s no reason for this function to exist. Can you see why?

If I was gonna make a suggestion, it would be to use some formatting tool such as black to make sure your code is styled in a standard way.

I can give you some basic Python set-up advice (which is hard-won because nobody tells you this stuff):

1. Use pyproject.toml, not requirements.txt. It’s better and it’s the recommended way.
2. Always use type annotations. I saw you used it a bit - well done! But just use it everywhere. Add declarations & type annotations for your class member variables. You’ll thank me later! Also prefer Pyright to Mypy; it is far far better (and the default for VSCode).
3. I would recommend using Black to autoformat your code. It’s easily the best Python formatter. You can automate it using pre-commit.
4. ALWAYS PUT MAIN IN A FUNCTION. You should not run any code at a global scope. Do it like this:

def main():
  ...

if __name__ == "__main__":
    main()

There are two reasons:

1. Generally it’s good to keep variables to the smallest scope possible. Putting everything in a global scope will just mean people start using it in their functions, and then you have a mess.
2. Any code you put in a global scope gets run when you import a module. Importing a module should not have any side effects.

Other minor things:

1. Path has a crazy/neat override of the / operator so you can do Path("foo") / "bar" / "baz.txt".
2. Don’t use assert for stuff that the user might reasonably do (like not choosing a background image). assert is meant to be for things that you know are true. You are asserting them. Like “hey Python, this is definitely the case”.
3. TK is an ancient shitty GUI toolkit. I would recommend QT instead (though it will probably be more pain to set up).