First of all, your original image should contain EXIF data that shows when and with what camera it was taken, right down to the serial number of the camera itself. If you "load" the info into the camera, it can also record your name on every image. And, if you are careful *not* to use "save for web" in Photoshop (or similar), which strips off EXIF data, that might even still be retained in the image you uploaded, possibly even the one they are using. At any rate, having the original is all the proof you need. It's obviously the same image.
The next thing to consider is how long it's been since you posted the image online the very first time. If it's less than three months since "first publication", it is more worthwhile to pursue "damages", and your next step is to immediately register your image with the US Copyright office. There's info how to do that on their website. Note, you can register a very large number of images at one time, for one fee.
If it' ls been longer than three months since first publication (the act of putting it on the Internet constitutes "publication") there's less reason to strongly pursue the case. There is simply less money to be gained. However, you can still bill them for their usage of your image, offering them a license with limited usage terms along with an invoice for the amount due to you. You just need to determine what a "typical fee" for the image usage would be, and how long you wish to license the image.
Along with the proposed image license agreement and invoice, or perhaps even in advance of it, send a "cease and desist" letter, thanking them for their interest in your photography, but informing them that they are infringing on your copyright and must either immediately remove the image from their website or come to terms with you under the image licensing agreement and make prompt payment to your for the usage.
You already took a screenshot of their website, which, along with the original image you have, is very clear proof that they are infringing upon your copyright. Keep the screenshot, check if they are using the image on multiple pages within their site, too, which might up the ante a bit.
If you are unsure how long they have been using the image, perhaps you could try looking at the source for the page or try researching it with the "Wayback Machine" (Google for that), which shows historical versions of websites.
Your billing invoice should include a time limit of, say, 30 days to remit payment for usage of the image. If they fail to do so, and fail to remove the image from their site, you have a case to pursue them legally. If it's been less than 3 months since first publication, that could be big bucks. If longer, it's a lot less and more of a small claims court issue.
