go to
http://imageshack.us/
click browse select the picture hit host
once it comes up go to the very bottum hit advanced select that url it shows and copy it
come back here create the thread you want select the image icon then past the url into the back that pops up(make sure not to double post the http things that is in there)
that is your 30 sec guide to image hosting or you can just copy and paste the url's for thumbnails for forums but those get annoying and don't always work