If I understand what you want to do - you can just delete and item from a message thread - press and hold on the photo and select more from the list of things you can do and select delete - to get rid right way select edit from messages and look at deleted messages and delete them
Yeah I just discovered that afterwards. Trying to delete all of them at once crashed the messages app on my Mac. Even though I had multiple selected on the Mac, it was only deleting one at a time.
And doing it on the iPhone locked the app up, so I had to do a few hundred at a time, still locking the app up though force closing and reopening shows the photos/videos were actually deleted.
Luckily I have an external keyboard for my iPhone, holding shift to select a lot at once has saved me from tapping the screen a few thousand times.
Edit: Do this by tapping on the person’s name up top, scroll down to photos and tap see all. Then you can select one photo at a time, or with a keyboard, hold shift to select multiple.
Then tap on “Download images from the cloud”, it’ll download 20-30 and rinse and repeat deleting more.
No easy way to delete all of them at once without deleting the whole message thread.