Ever since Google+'s web interface was revamped, one function has been absent from the new redesign: image drag-and-drop. This meant that you had to click on the image picker, browse through a file system, then choose the image you want to insert into a new post or a comment. It's especially counter-productive when you're already flipping through your images and you think you've found one that you want to share on G+.
Now that function is back "by very popular demand," which is easy to understand. Googler John Nack explains where this option is functional now:
● Drag photo(s) into any stream to which you can post in order to start a new post with the photos attached
● Drag photo(s) into a post box to attach them
● Drag photo(s) into the image picker (inside of a post box) to attach them
● Drag a photo (singular) onto a comment to attach it to the comment
The option isn't yet live for everyone, so you may need to be a bit patient if it doesn't work for you yet. For example, Artem can't drag photos into the post box without opening the image picker first (i.e. the third bullet works for him, but the second doesn't yet), which is his most frequent use case for it. All of them work for me, though. I guess #Artemsluck claimed another victory.