Tim,
first you have to go into the proper 'post reply' link.
find an image you want (it has to be hosted on a webpage)
right click on the image - and then click on properties
highlight and copy the the part that says 'adress (url) - it must start with
http://
paste that into the main message body section of your reply
highlight that again, and click on the button near the top that says 'Img' - it should come up with a

at the end
then preview it to check it works