There is a fifth option that is related to these. Here is how I would play these demands:
First, look around and see if there is the exact same image offered elsewhere for license. If it is, point it out to them and tell them to buzz off. Second, find out the actual going rate for that "rights managed" image. It will be fairly high, but you should see what that is. THEN, find similar royalty free stock images. These are often prices at around $10 to $50. Document a bunch for your records.
Compose a letter to the stock image company. Without any admission of guilt, explain that you simply do not have time to research their claim on this image and give them a low-ball offer. (Maybe $50 more then you offered.) Put a deadline on it, giving them 30 days to respond. Tell them if they pursue this
you will need poof of a registered copyright and if they are not the original copyright holder, to please provide a chain of title to this image. Also tell them that since this is the value they are claiming for this image you will require proof of how many times this "rights managed" image was licensed and on what web site and for what price.
So far they have been rejecting these offers, but I predict the day is approaching where they will accept them. If they don't you have to white-knuckle it and hope they don't file suit for 3 years. But even if they do, you have documented evidence that you tried to make a fair offer and you have examples of other images that sell for much less.
In any case, you probably want to go through your site and replace any images you "found on Google" with images in the public domain or images you have taken yourself.
Good luck.