I'd suggest a polite phone call to their design department, or image buyer (should they have one). Either that, or an e-mail if you'd like to keep an audit trail.
If they have used an image in good faith (in their own mind), there won't be an issue for them to give you their source. Especially so if the image has your name, details etc within the metadata.
If that doesn't work, I'm guessing a letter to their legal department.