Find and Add Photos
In the absence of an iMessage App Store, the last option you’ll find on the “Apps’ menu is a “Photos” button that lets you quickly drop in a picture from the photo library on your Mac.
It’s not limited to just browning for recent photos, however; all the same search logic that you can use in the main Photos app is available from the search field at the top, meaning you can quickly narrow things down to people, places, or things, leveraging the powerful image recognition technology that Apple has built into the Photos app to help quickly find something.