That is so maddening, and is the number one reason why I stopped using the stock messaging app.
One solution is that the slide show can actually be paused if you tap the screen, and then you can manually skip to the next photo with the onscreen control. Perhaps a better option, though, is to install a third party messaging app from the play store, such as Google messenger, which properly displays multiple photos in mms messages and has none of the other bugs in the stock messaging app.
After you install a third party app, go into settings / more... / default sms app.
You can try the app out and, even if you don't like it, uninstall and switch back to the stock app without losing any messages - they will still be in the messaging app, including the ones received by Google Messenger.
https://forums.androidcentral.com/e?...token=Xd-PEDPe