With iOS12 decorate your photos using Camera App from iMessage

Opening the camera App from iMessage in iOS 12 will let you add stickers, text, shapes, Animoji and Memoji to your photos. Go to an iMessage thread or New Message screen and tap the camera App. Once the camera App opens, tap the star icon in the lower left corner to bring up the options available to add to your photo. By tapping a particular option will show the choices to pick and add to the photo.

Once you added your choice, close the choices screen by tapping on the 'X' then the camera shutter button will be visible to take the picture with the decors added. After taking the photo, you can embed that into the iMessage by clicking the blue arrow.