I don't really see the issue here. You missused a photo, knowingly or not, you still did. Plead for leniency maybe but I don't believe you have any right (legal or otherwise) to try and say that you shouldn't have to pay this.

As someone who has had my own images used in completely inappropriate places I am a firm believer in either buying or asking permission from the owner for the right to use the image.

Randomly getting images from google image search and using them is stupid and irresponsible, and is obviously going to cost you time and energy to resolve.