Copyright is complex even in a single jurisdiction. My broad understanding is that *generally* copyright expires because a specific period has passed, not because the rightful owner has simply done nothing about it for 10 years.
Even if copyright for the images expired tomorrow it's likely they would be able to go after you for your infringement while it was valid.
It's not like a trademark where you may have to proactively defend your right to it.
I do agree that the waters are a little more muddied if you were selling on their behalf (ie your sales make them $) and they were fully aware of your use of the images all this time. But as a black and white answer, it's likely they do have copyright on those images, and unless you have an agreement where they've explicitly or implicitly (eg through ToS) authorised you to use those images they can legally pursue you for infringement.
If they're asking for a financial settlement then you're better off contacting a lawyer specialising in copyright.