Before iOS 12, if you wanted to share a photo, you’d tap the camera button in a Messages chat and select the most recent photo you’ve taken or you could select an older photo. In iOS 12, Apple changed things so tapping the camera button only lets you take a fresh photo. To find and send a photo that’s already in Photos, you use the Photos mini-app in Messages. This looks like the Photos icon in the tray. If you do not see the tray with the Photos icon, as well as other icons, tap the Apps button to the left of the message field to show the Messages apps, and then tap the Photos button to see a list of recent photos. Tap one or more to add them to the message, and you’re ready to send! This also works with the iPad!