That is something that stumped me for ages.
the way i do it is to have some web space online where i store pictures etc and refere to that
you can use some ubb code and put the link to the picture in image taggs
It looks sometihng like this
[ i m g ] w w w . picture address\picturename.jpg [ / i m g]
with no spaces
but if you have a look at the add bbc tags when you post your message the second icon on the bottom row is for posting images, and that should help you out, you just have to put the link to the picture between the two centermost square brackets