
You
can post images directly from facebook (as I did above

) - the trick is to get the image up on your browser as the main photo showing, and then right-click on it and choose "copy image location" (or whatever your browser's equivalent is) from the contextual menu.
Then you paste that copied url between the img /img tags in your post here (as has been explained by others above), and the image will display in your post.
Oh, PS: I didn't choose that image as a comment on previous posts, it's just the first one that came up in my facebook Wall Posts gallery, and one of my favourites from XKCD

.