You need to copy the address of the photo from the photo host site, paste it here, highlight it, then click on the IMG tag above the text box. IT can be tricky the fiorst time, but once you get the hang of it, it's a breeze. :D You can generally (there are some exceptions) right click/copy/paste the address of any image you see online for sharing here. Sometimes the photo is garded, or too big, but usually it works. Just need to surround the address with the IMG tags. like this except without the space inside the first IMG tag:
[i mg]http://pic20.picturetrail.com/VOL91/512063/21041131/344523371.jpg[/img]