On the toolbar when you are making a post is a little picture of two mountains with a sun behind them (it's yellow). Click on that, and then copy the URL of the picture into the dialogue box. Or just follow this format:
[img*]URL[/img*] - but remove both *
To get the URL you have to upload the picture to the internet (there are many sites to do this for free... I like www.flickr.com
Right click on the picture, select properties, and copy the URL. Paste that into the dialogue box or between your IMG-tags.