In the text editor, click on this button:

A new window will open:

Here you should type the url (address) of the picture you want to include.
EXTRA NOTES:
1. Please avoid linking to copyrighted photos from other websites. It is called hotlinking and it is a practice to avoid.
2. Linking to a photo instead of adding it as an attachment to your message can be convenient if you want to include a picture which is bigger than 500KB. What you can do in this case, is to upload (transfer) the photo from your computer to an image hosting website, like http://www.imageshack.us/ .

You don't need to register or pay any fees, although (free) registration will give you extra advantages. The Firefox toolbar from Imageshack is very convenient too, if you plan to add photos frequently. Adding a photo is easy and when the file transfer will be succesfully finished, you will get the code that you need to add in your message. This will automatically show a clickable thumbnail of your photo in your message.