Just use the little box "Choose File" just above the box your type you message in. It can be used repeatably to do multiple images. If you hit return after each image is selected they will appear on separate lines. If you just do multiple file selection without a return they will appear side by side if they are small enough. Just use the "Preview" box to see if you are happy with the way they look and then use the "Post" box to create the posting.
One above the other...

Side by side....