Upload your image to an image hosting site. I like
www.imgur.com, but there are dozens of other sites that work. Take the image link from there, and then either click the photo button and follow the directions (choose from URL) or manually add tags that look like this, but with the dashes removed:
[-IMG-]http://i.imgur.com/XaxFz5s.jpg[-/IMG-]
Minus the dashes inside the brackets, that displays like this: