As soon as you find ripped-off articles or photos, you need to start collecting evidence as though you were going to court. It'll help in negotiation, even if you don't get to court.
Save everything you find, making sure that you record the date and time you found it. For this reason a printout is still better than a screen grab, if harder on the forest. If you know how, you could make a screen grab and add your own date and time, but unscrupulous pirates (and is there any other kind?) know they could get lawyers to quibble - so we won't explain to those who don't know.
How long has it been there?
Copy and paste the URL of the page you have found into the "WayBack machine" at web.archive.org to look for earlier versions of the same page. This allows you, with some patience, to find out how long the rip-off has been posted on the web. Save or print out the earliest version you can find.
Note that you cannot search web.archive.org for content - text or pictures. You have to find the URL (web address) of a page, and then find that in the archive. The archive system appears to wait a few months before displaying pages that it has saved. It may not save every page from a website every time it visits some of that site. It often fails to save copies of pictures - but you can often see where they were on the page.
Text © Mike Holderness & previous contributors; Moral rights asserted. The collection (database right) © National Union of Journalists. Comments to email@example.com please. You may find the glossary helpful.
The National Union of Journalists must not, can not and would not wish to dictate rates or terms of engagement to members or to editors. The information presented here is for guidance and as an aid to equitable negotiation only.
Suggestions apply to contracts governed by UK law only. In any event, nothing here should be construed as legal advice.