Hi, Roger Allum
Thank you for explaining. One correction to the AI's answer: the newer Photos app does support slideshows, so you don’t necessarily need another app.
If you’re viewing the pictures in Photos, try starting the slideshow from inside the app:
Open Start, type Photos, and open it.
Open your picture folder from the left side. If it isn’t listed, select Gallery, then Add folder to add it.
Open one picture and press F5 to start the slideshow. Try this rather than selecting all the files in File Explorer.
If it still repeats the same 30 pictures, my next suggestion is to repair Photos. Open Settings > Apps > Installed apps, find Microsoft Photos, select the three dots beside it, then Advanced options > Repair, if available. Afterward, reopen Photos and test again. Repair keeps the app’s data.
If it still happens, are you watching the pictures in a full-screen window, or as your desktop background?