I often post the same picture to multiple sites, to upload to each site would be a major PIA. It is so simple to keep all my pics on a site like photobucket, then simply paste the link to what ever I am posting about.
It isn't rocket science.
There is the added advantage that people see my pictures as full size, not a thumbnail that you then have to click on to see the image.
It also saves the forum owner money by not having to use all of that bandwidth to host all of those pictures, it helps keep forums free.