Copyright infringement
Why not contact them yourself, explain why it's your image and how you prove it*, point out the error of their ways, set out the consequences of their actions unless they cease and desist (or settle), name a price, then see what happens? Give them 14 days.
If they don't settle the matter, unleash the lawyer!
*It's a cityscape at night, which anyone could have shot. What is unique about it that proves beyond reasonable doubt that it's your image, and why/how do you own the copyright?
but any advice from anyone with experience of this type of thing on how best to proceed would be welcome
Yes I've had this happen several times.
My advice is is to take it as a compliment you take pics good enough to be "stolen" and rather than get angry and lose brain cells over it, instead feel good that your images are worth you attention.

I've worked and lived in the Middle east and know that copyright means nothing there. You should know this. Don't worry about it.

[i]Bodies: 1x K-5IIs, 2x K-5, Sony TX-5, Nokia 808
Lenses: Pentax DA 10-17mm ED(IF) Fish Eye, Pentax DA 14mm f/2.8, Sigma 17-70mm f/2.8, Pentax-A 28mm f/2.8, Sigma 30mm F1.4 EX DC, Pentax-A 50mm f/1.2, Pentax-A 50mm f/1.4, Pentax-FA 50mm f/1.4, Pentax-A 50mm f/1.7, Pentax DA* 50-135mm f/2.8, Sigma 135-400mm APO DG, and more ..
Flash: AF-540FGZ, Vivitar 283
The legal route will cost time, money and energy and you may end up with a bill and little else. On the other hand the end result may end in your favour.
Fletcher8.
1) ask them to show you the licence they have to use the image. This gets them engaged and might show further usage.
2) send them an invoice using Getty calculator to calculate the licence fee and explain you are willing to accept that as payment. Payment is expected within 7 days failure to pay will result in court proceedings etc where you will be pursuing costs for copyright infringement plus the original fee.
3) if they don't pay unleash the lawyers there are lots of copyright lawyers now working on no win no fee the one I use here in the uk charges just 25% of any winnings.
PPG Wedding photography Flickr
Concert photography
Currently on a Pentax hiatus until an FF Pentax is released

PPG Wedding photography Flickr
Concert photography
Currently on a Pentax hiatus until an FF Pentax is released

Ken
“We must avoid however, snapping away, shooting quickly and without thought, overloading ourselves with unnecessary images that clutter our memory and diminish the clarity of the whole.” - Henri Cartier-Bresson -
but any advice from anyone with experience of this type of thing on how best to proceed would be welcome
Yes I've had this happen several times.
My advice is is to take it as a compliment you take pics good enough to be "stolen" and rather than get angry and lose brain cells over it, instead feel good that your images are worth you attention.

I've worked and lived in the Middle east and know that copyright means nothing there. You should know this. Don't worry about it.

It's all very well for amateur photographers like yourself to be so generous but there are some of us who actually earn their livings from photography and others who take a higher moral standpoint on what is essentially theft.
Ken
“We must avoid however, snapping away, shooting quickly and without thought, overloading ourselves with unnecessary images that clutter our memory and diminish the clarity of the whole.” - Henri Cartier-Bresson -
but any advice from anyone with experience of this type of thing on how best to proceed would be welcome
Yes I've had this happen several times.
My advice is is to take it as a compliment you take pics good enough to be "stolen" and rather than get angry and lose brain cells over it, instead feel good that your images are worth you attention.

I've worked and lived in the Middle east and know that copyright means nothing there. You should know this. Don't worry about it.

It's all very well for amateur photographers like yourself to be so generous but there are some of us who actually earn their livings from photography and others who take a higher moral standpoint on what is essentially theft.
Actually my moral standpoint on what happens to my images is no lower than anyone else's moral standpoint on what happens to their images. I was just giving advice based upon on what I do when I discover someone has used one of my images in response to Alien's request for advice in the OP.
He's going to get a range of opinions. he hasn't chosen to take my advice and that's fine
[i]Bodies: 1x K-5IIs, 2x K-5, Sony TX-5, Nokia 808
Lenses: Pentax DA 10-17mm ED(IF) Fish Eye, Pentax DA 14mm f/2.8, Sigma 17-70mm f/2.8, Pentax-A 28mm f/2.8, Sigma 30mm F1.4 EX DC, Pentax-A 50mm f/1.2, Pentax-A 50mm f/1.4, Pentax-FA 50mm f/1.4, Pentax-A 50mm f/1.7, Pentax DA* 50-135mm f/2.8, Sigma 135-400mm APO DG, and more ..
Flash: AF-540FGZ, Vivitar 283
Though I don't consider myself a professional photographer, it's paid for all my equipment and a holiday, and provides an erratic but steady trickle through stock and print sales. I don't expect free services from anyone else, and neither should they expect them from me.
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Here, there, everywhere
Link to my original image here link
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