On the upper right hand side of the Post box, you'll see an Img button. Use it to insert the URL of the picture.
Let's say the URL of the picture is http://www.domain.com/subdirectory/image.jpg
You have to type (without spaces in the img tags):
code:
[ img ]http://www.domain.com/subdirectory/image.jpg[/ img ]
The above piece of information was brought to you courtesy of Kenneth Tan Jian Wei.
