Type hints of `convert_image` is not correct.

For now the type hints of `convert_image` is:
```python
def convert_image(
    src: pathlib.Path, dst: pathlib.Path, **params: Optional[dict]
) -> None:
```
But we can use `io.BytesIO()` as src and dst. 
Also, When I try to put the `fmt="PNG"` as a parameter, the Pyright will complain : `Type "Literal['PNG']" cannot be assigned to type "dict[Unknown, Unknown] | None`