Right click on your small image, and choose " Open Link in new Tab".
That'll bring up the postimage.org website, which is the picture hosting website you're using. Right click on the full sized picture in that new tab and choose "copy image address".
Then in a reply box on 24 hour campfire, post the image address between [img] and [/img].
There may be an easier way to do it, but my way does work.
