Right-clicking your pictures and choosing Copy Image, then pressing Cmd-V in the Discourse editor produces:
<img src='https://discuss.nubits.com/uploads/default/original/2X/7/756f38aca1ea2dc965137e3cfd4c3b994352906b.jpg'>
<img src='http://memes.doublie.com/wp-content/uploads/2015/07/bottlecaps.jpg'>
URL depending on if Discourse has had time to cache the picture.
Same with right-click > View Image, then Cmd-A, Cmd-C, Cmd-V.
Try pressing Ctrl-Shift-Cmd-4 and selecting a part of the screen with your mouse, then go to Imgur and press Cmd-V. For me that inserts the picture in clipboard and lets me upload it. This is how I imagine Discourse previously worked for @masterOfDisaster.
Makes me wonder how Copy Image functions.