With macOS 10.14 Mojave, Apple has beefed up the Mac’s privacy so it more closely resembles privacy in iOS. You’ve noticed that when you launch a new app on your iPhone or iPad, it often prompts for access to your photos or contacts, the camera or microphone, and more. The idea behind those prompts is that you should always be aware of how a particular app can access your personal data or features of your device. You might not want to let some new game thumb through your photos or record your voice.
Before iOS 12, you’d tap the camera button in a Messages chat in order to share either a brand-new photo or a photo that had already been taken. In iOS 12, Apple changed things so tapping the camera button only lets you take a fresh photo. Read more about changes to Photo Messages.