It depends on where the picture comes from:
-if it comes from your computer, there is, at the bottom of the "post reply" page, an "Attach file:" section. Just click on the 'browse' button,...
-if it comes from the internet, you can use the: [ img] address of your pic [/img ] tag (without space between the [] ). (In this case, you can also use the first method by downloading the pic to your computer first)
PS : Don't forget to go & see the FAQ first when you have some questions... http://www.killermovies.com/forums/misc.php?s=&action=faq