If the picture is on your Computer ..........upload your picture at some of the free Sites http://www.imageshack.us/ .............then copy the address form ther and use the img tags to hotlink it
Or if it already at some web site simpley the address of the site withing Site URL here [ /img] tags will do .
Here is a image i hotlinked Press the Quote to see how :
[img]http://www.groovy-chick.com/peace/peace13.jpg