There's a text field for entering a path to an image to use as puzzle. Add a Browse button to make it actually useful.
We'll strip the file:// prefix as the image handler cannot cope with remote images anyway.

The text field isn't disabled when the checkbox is unchecked, instead, it's smarter: as soon as you start typing in the text field or click the browse button, it's automatically checked. Saves you a enable checkbox and then press browse dance. (We should use this approach more imho)