You need to have the tags arranged like this:
img - surrounded by square brackets
the url address with no spaces after the square bracket
'/img' - surrounded by square brackets with no space in front of the first square bracket.
like this, except the square brackets face inward:
]img]http://your.own.address/yourpic.gif]/img]
and if the line wraps into the next line on the screen, keep typing and it will interpret it correctly, if there are no spaces.
hope that helps