auto save photos in imessage

The simple answer is you can't turn it off, under iOS 11 if you took a pic in Messages it was attached and sent without going to Photos, iOS 12 changed that and it's now automatically sent to Photos as a JPEG but with location data stripped out.

Your not alone in thinking this is a bad move especially if your device doesn't have suitable storage capacity, at least the old way gave you the choice depending on how you took the pic, the only option for the user at the moment is to delete the pic manually.

You could send Feedback to Apple requesting a user controlled option though.

Feedback - iPhone - Apple

Have you tried uTube video and ask that question and a video pop up and just listen to what the video is saying and take notes too. If you must you can play the video over and over again until you get it right this time. Be sure to click on the 'subscribe button' below because the person who made the video gets paid for everytime the video is played.