1) move your mouse onto the image.
2) click onto the image (once)
3) when the image is selected, click on the link icon in the editor (it looks like a sideways 8 with a little plus sign)
4) a pop-up window will appear once you have clicked on the editor icon
5) enter URL beginning with http:// (that will tell Joomla that the address is external) E.G.:
www.shakespearesglobe.com/
6) ignore Link Browser
7) ignore anchor
8) select target: Open in new window
9) Title: give it a descriptive title, e.g.: visit the Shakespeare's Globe website (external link)
10) click on the update button, and the pop-up window will disappear.
11) click on apply (if you want to continue working with the article or save if you want to close the article.