The basic answer is, if they didn’t buy it from a stock site, then an image isn’t a stock image - and if it was a stock image they’d definitely pay for it.
Many people think the internet is a supply of free images - it’s not. They are breaking the law - stealing, because that is what they are doing, any image is a breach of the 1988 Copyright, Designs and Patents Act, which is a criminal, not a civil offence. As with any law in the UK, ignorance is no excuse for breaking it.
My husband is a commercial photographer and I run the business and - like many photographers - we have an ongoing problem with breach of copyright. Initially I policed it, but now it’s got so bad and I found it so frustrating and upsetting (remember, these images are our living and so put food on our table!) that we now use an agency to pursue it for us. The last one they advised us they were chasing has been sent a bill for £800 !
If anyone finds their copyright being breached the first thing they should do is send a cease and desist letter asserting their copyright (which in law is yours the minute you click the shutter), stating exactly why it belongs to you with dates when you took the photo and any proof of where it is used. Then demand instant removal of the image. If you are feeling particularly aggrieved or brave you can accompany this with an invoice for the use of the image they’ve already had - don’t go mad, I’d say £25- £50 would be reasonable. In my opinion it’s important to put some sort of figure on it to make them realise there’s an actual cost to the photo - it might make them think twice about stealing someone else’s. Just be prepared to not get paid
If you want me to send a template, or if anyone thinks it’d be useful for me to publish it someone on the forums, let me know.
Oh - and I agree with Sasha @SashaGarrett - watermaking isn’t particularly attractive and can be removed, but subtle watermarking can be a gentle reminder to the casual abuser.