Firstly, don't use copyrighted material without the authorization. Now, once someone has decided that you are infringing on their copyright, it is incumbent upon them to notify you. You do not simply owe them money. However, if your use of their copyrighted material has somehow cost them in the way of lost business or other expenses, they may sue you.

If you happen to be a client of theirs, that could be a different story. If you have entered into an agreement (contract) and you violate that agreement, they may bill you per the contract.

My advice is not use images without the proper authorization. If you are caught, I would further advise you to remove the image immediately to mitigate any damages. Then I would wait for them to start legal action. Your defense, if it came to that, would be ignorance/stupidity and the fact that once they advised you of the infringement, you complied by removing the image.
__________________

__________________
DrTandem's San Diego Web Page Design, drtandem.com