Yahoo! doesnt allow quick linking so don't bother with anything from there. Find a pic you want, right click it and select "properties" copy the URL then paste it between the img tags like Boi said.
You'll feel like a jerk when realize how silly/stupid it is to post pics.
First of all you need a pic to use. This pic has to have a web URL, like "http://www.bonnevilleclub.com/forum/templates/bonndef/images/logo_bonn.gif". Once you have that, push the IMG button ONCE and ONCE only. All this does is put the [img] tag down, then paste the address for the pic, and click the button again. You can also type it in, but the buttons are there, and they work the same.