Registering your copyright will not prevent people from taking and using your images. It gives you stronger recourse when that happens.
What you should do first with the present usage is send an email or written notice to the user to "cease and desist" use of your copyrighted image (all images are automatically copyrighted, the moment you take them). Then if they fail to comply, contact the website moderator or administrator. If the website administration doesn't respond, there's further legal action you can take online.
It really doesn't matter where they acquired the image and you only need to register it once. You don't have to track down every usage you've made of the image and individually register each of them.
The image was effectively "published" when you displayed it on 500x. If that was done more than 90 days ago, registering your copyright now would give you somewhat limited recourse. But registered is always better than unregistered. Visit the U.S. Copyright office website for more info (
https://copyright.gov/).
You can register a large number of images at one time and for one fee.... Basically you just need to submit "thumbnails" of all images that are large enough to identify the image. This is mostly done online now, but it's also possible to do by mail.
Much as you might hate them, you should put a watermark on your images to help protect them. In fact, I make my watermarks an advertisement of my web galleries. I have hundreds of images "stolen" and used on Facebook and other sites every year and don't worry about it, since they are watermarked. It's not like anyone is profiting off that usage.... and they're effectively giving me free advertising when they display my image with the watermark. I also keep all images displayed online relatively small... no more than 700 or 800 pixels on the longest side. This helps prevent misuse, too.
I also embed copyright ownership information in the EXIF. But that gets stripped off all the time. Some sites do it automatically whenever images are uploaded there. Some image editing and optimization software strips EXIF too, when you "save for the web".
Watermarks and copyright info embedded in the EXIF are "copyright protections". When an image is registered, should there ever be a law suit over misuse of the image, the court can award up to $30,000 per instance for intentional removal of copyright protections. With registered images, this is in addition to other awards the court can make such as usage fees, penalties and recovery of legal costs. Because this can amount to tens of thousands of dollars when images are properly registered, suits over copyright infringement are heard in federal court and intellectual property attorneys will take cases on a contingency basis.
Unregistered images, on the other hand, can only sue for "fair market usage fees". Because the $ amounts don't amount to much in most cases, copyright suits for unregistered images will typically only be heard in small claims court and you'll likely not have legal counsel unless you pay for it yourself out of your pocket. There's no recovering legal fees and court costs with unregistered images. Also no penalties for misuse over and above the "usage fees".
I ain't no attorney... just a photographer. You should consult a knowledgeable intellectual property attorney for further info and advice.