Hi,
Messages on the Mac was created by adding the iMessages Account to iChat.
iChat was at version 6 at the time and Messages continued the Numbering becoming version 7 in OS X 10.8
The way of adding a picture of any sort was to drag it from the Finder window that is open to the Text Entry Field and when sent it is in it's own balloon.
At one time text added at the same time was included in the same balloon but this does not happen in later versions.
Basically you have two apps that have the same name on two different OSes that happen to be compatible in sending and receiving iMessages.
It's a bit like Office on a Mac and PC both can read Word documents.
There are slight differences at certain points which can mean some things are done differently.
However even though I have never used this Keystroke they is this in the Buddies Menu

Pics are Files as far as the app is concerned and it works in an iMessages chat.

7:48 pm Sunday; February 19, 2017
iMac 2.5Ghz i5 2011 (Sierra)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone 6 iOS 10.x and an iPad (2)