1. Do as many suggest and make sure you have an archived, unimpeachable copy of the offending usage/site BEFORE uttering anything to anyone - unless you simply want usage to stop and no $$. But will happen again. Too many folks think on internet means 'free' and others imply know how to work the system.
2. Go the polite, "you made an inadvertent boo-boo" no-accusatory way first, showing evidence and while you understand they made an 'innocent' (no need to get anyone's back up) mistake using the images, make it apparent they are chatting with a polite but firm and well informed pro who knows how to resolve the situation reasonably. In short, avoid the "give me $$ for my pictures you thieving idiots/scum or I'll sue..." approach.
2a. If they simply apologize, remove images and go quiet on $$ or simply suggest attribution vs $$$, think long and hard and unemotionally about any next steps.
3. Don't mention or even hint at lawyers unless you REALLY know what you're walking into. The time-honored tactic for someone being sued or who thinks they are going to be sued is to erase evidence and simply go silent until such time as they actually have no choice but respond (e.g. receive a summons, etc., - if they ever get one). No responding emails, no layers letters back, nada. You send your spendy letters letters/emails into a black hole.
The idea is for you to spend lots and lots of legal $$$$ before the process even gets started. You whittle away $$ and get a VERY real sense of dread with every $1000 3- paragraph letter your lawyer gleefully pens of what the process is going to involve in $$ and time just to get to court, let alone thru court - if it even gets there. The only thing achieved is that your lawyer makes insane money for the effort involved, you lose time, money and resolve and they don't even break a sweat.
The sad reality of the system is that suing is a matter of looking at the cold hard math. Principal is all well and good in theory (like Socialism), but what is this going to cost me vs. what I will get back - or avoid losing (in usage fees). Does the logic for moving fwd top add up? As a simplistic example, spending $10,000 to get $10,000 back may give you a sense of satisfaction, but who is the actual winner in the end?