The best course of action would be to try to work it out with her directly. Before doing anything, capture the use of the image on the website. Then call her and get her side of the story, then tell her what the 'new' fee is. I'd give her 3x or more over your initial quote for the infringement.
If she balks, just politely inform her the images are copyrighted, and by pursuing it through legal channels you will be asking for the same amount, but also to have your attorneys fees covered. Point her to the copyright law so she can see the damages and awards for willful infringement. Let her know that you don't want a hassle, however without a written grant of rights from you she can not prove she didn't infringe on your copyright. Therefore, there is a very high probability she will end up paying the fees, your attorneys fees, and possibly statutory damages if the judge so sees fit. Hopefully she will see she is not in the best of positions, and try to work something out.
If that fails, then you have to decide if its worth it to hire an attorney for $100-$300 dollars, and have them draft giving her a chance to clear it up prior to filing for infringement, and asking her attorney to contact your attorney ASAP. It should also state the same amount, with the attorneys fees now added as well. Hopefully, she will see you are serious, and try to negotiate her way out.
If not, it's probably best to write it off. Trying to take it to court for the amount in question would be too costly. You could spent $10K trying to collect a couple hundred dollars, and even if you won, there is no guarantee you will ever be able to collect. Working it out as best you can outside the legal system is the best avenue to take IMO.
Another avenue if you don't get satisfaction, is to get a DMCA take down issued on her website. That will at least stop her from using the image if nothing else.