You need to host it somewhere on the Web. Then just insert the link into your post. If you want it to display directly, youll need to parse the link with image tags like this: [im
g]http://www.weblink-of-my-picture.com[/img].
Free webhosting for small files can be done on
http://www.picturehangar.com - otherwise, just google for free webhosting... lots of sites out there that offer.