When using postimg to post pics, once you upload your pic to postimg and the page changes to the preview screen, all you have to do is copy the address under or beside the "Thumbnail for forums:" option, then paste it here
Or, click the icon to the right of the address (this will automatically copy the address for you). Then paste it here.

This will post the thumbnail of the pic here. Members can click on the thumbnail to see the full size pic.