This image of a tiger cub has an internet address or url, it's http://i40.tinypic.com/2n67wk8.jpg
Putting that url between [img][/img] will display that image on a forum like this.
Also it helps if its an image from a site that allows you to link it like tinypic, you can also upload pics there,then put their urls between [img][/img] to display them here.