easy:
step 1 -> find picture you want to post on net somewhere
step 2 -> copy down link to picture
step 3 -> hit reply to post
step 4 -> hit the IMG thingy
step 5 -> type in link to picture
step 5.1 -> add blabla (optional)
step 6 -> hit submit reply
done.
if nobody has the pic you'll need your own webspace.
regards