Hi Oleg
It depends on a couple of things - where the ad was created and where it was used. Regardless of the "value" of the use, had they approached a stock agency, they would have been charged a substantial usage rate. Somewhere in the $3k to $10K depending on the print run, if it was used online, or if it is used in an outdoor display ad.
Usually in the States, retroactive licensing is widely upheld as 4 to 5 times what you would have licensed the image for (in an acceptable purpose) in the same type of ad.
I've had a lawyer/producer shmuck use a few images of mine without permission, lets just say I made him bleed, and I bought a lot of toys with the settlement.
You need a Lawyer that specializes in Intellectual Equity. You have evidence from your server logs on your site, find out who was on the website, connect the dots, and cha ching. It would be really funny to watch a bunch of gun grabbers buy you a boat load of new guns!
Edit: I wanted to add that since IANSA has a US office, and this was used on the internet, you may be able to go after their office in the States. Regardless of the laws of the country of origin, it was shown on US computers, over the net, and originated from a server undoubtedly based in the states. Go git em.
LH