The real answer to that question is: "It's worth pursuing if I feel like it's worth pursuing."
You need to decide if it's worth the trouble to ask them to either pay for the use of it, or remove it from their promotion page. You will almost certainly arrive at some sort of resolution without involving a lawyer.
So what do you want from this situation? Do you want payment? Acknowledgment/credit? Do you just want it removed? What would be the most satisfying outcome for you? You can see the various reactions some of us on the forum have about this kind of copyright infringement/IP theft (and make no mistake, this is theft) but only you can decide for yourself what you want to see happen and how you feel about it.
I've dealt with this exact scenario many times over 20 years and one thing I can tell you is this: if you express your displeasure to them and inform them that they are using your intellectual property on their site without permission, payment or attribution, the most likely result is that they remove the photo fairly promptly, and most often it happens without any response from them. IE: don't expect an apology. You can bill them for the usage rights, but it's unlikely they will pay to continue using it (I had the company actually pay such an invoice only twice in 20 years). They might send you a two or three sentence apology, and chances are, they will make an excuse for their infringement, like "Oh, sorry, We just found it on the web and didn't see any copyright" or "Our ad agency provided the photo for us - we have no idea where it came from" or maybe "Our web designed told us he/she used only Fair Use materials they sourced". But there's a very good chance they will make an excuse, if they respond at all. Don't be afraid to tell them you will file a DMCA Takedown on them if they say they don't want to comply.