I just tried and you're right, I can't drag and drop from Google Photos.
Actually, what I do is I copy and paste the image from Google Photos. Open it up over there, right click the image and click on "Copy image" (NOT copy image address. Just "Copy Image").
Then over here, put the cursor in the text field for the reply, and hit Ctrl-V to paste. The lightbox tags quickly shows up after the upload is complete.
