Paper stole photo

You shouldn't have to watermark your photos, newspapers should know better.

Just sent an invoice and if it's not paid, try small claims court.

Is is a freebie paper or a regular newspaper? Where did they take the photo from and what licencing (if any) was attached?

Is this the same photo they've used twice? Are you certain you did not give them a licence to reuse the image when they paid you the first time?

Solicitor's letter?

Do they acknowledge/agree that it is your transaction?

And (as others ask) how did they get it?

You won't get much anywhere with this. The best thing (assuming you can prove the allegation) is to name & shame and let them take the reputational hit. You can do that openly and/or just mention to all your acquaintances. Local hire place did me once, refused to deal with their fraud and they lost a heap more business because of it than the few bob involved.

How do I go about suing the OP for waste of my time?

Consistently failing to answer the most basic question of "where did they get it from"

Why does it matter? Lets say the OP posted the photo on Facebook - do they lose the copyright on the photo? No, they granted Facebook a licence to reproduce it. The copyright stays with the OP.

Unless the OP uploaded it to a site which explicitly licences images to third parties (e.g. https://www.pexels.com/ - and I'm assuming the OP would know if they did that), the OP holds the copyright, and has every right to be paid for it.

Unfortunately, the cost of chasing up such a claim is likely to outweigh any potential award

The OP would know if they did that. But we wouldn't, unless the OP tells us. Which is why "where did the newspaper get the photo from?' is a relevant question, and a reasonable one to ask.

Not saying anything about the OP in this thread, but there is a noticeable phenomenon on Boards of people asking for advice or opinions, but only giving the facts which will tend to support the opinion they hope to hear. They're looking for reassurance that the view they hold, or want to hold, is correct. Guys, if you want to get advice or opinions that will actually be of any use to you, as opposed to just comforting you, you need to give all the relevant facts and circumstances, including the inconvenient ones and the ones that tell against you.

You actually answer your own question about why it matters.

It's a long shot, but if OP did upload it to a site like pexels.com and the newspaper took it from there, that would be significantly different to the newspaper taking it from Facebook or similar.

It would mean the newspaper wasn't actually obliged to pay the OP after first usage, and also that OP wouldn't be entitled to tell them not to use it again, since he/she had made it available on a royalty-free stock photo site, such that anybody in the world could use it in any way they saw fit, and as often as they liked.

Again, it's a long shot, but until or unless we're told otherwise, it's still a possibility.

It matters because if an OP refuses to answer a relevant question when asked multiple times its usually a good indication that they might not be sincere.