Say you wanted a picture from google like this.
Press on the picture so its big, then right click. Press 'Copy Image URL'.
Write this
[img][/img]
Then Paste the url in between the IMG codes like this
[img]http://upload.wikimedia.org/wikipedia/commons/thumb/2/26/YellowLabradorLooking_new.jpg/260px-YellowLabradorLooking_new.jpg[/img]
Wa LA!