I'm using a mac 2013 with high sierra.
I can receive photos in imessage but recipient can't see photos I send.
I'm just dropping the photos in the text area.
One recipient with an iphone and mac. While on facetime with my wife, I tell her that that I'm sending her photos on imessage. The photos sent do not open up on her iphone nor mac. Imessage photos she sends me of the kids open up fine on iphone and mac.