To embed an image from Flickr like the one above, you need to click on the photo and select the appropriate size (500px wide works best for this forum).
Right-click (or Control-click on a Mac) and Choose "Copy Image Location." (See the attached screenshot at the bottom.)
Then click on the "Insert Image" button (just below the Italics button) and paste (Ctrl + V) the image location between to two bracketed IMG tags.
**Updated** See also the (perhaps easier) instructions from below. (Thanks fcarlos.)