you can do it two ways.
1. insert the picture into the post, then go back click it and click the link button. it will ask for the url you want to link the image to.
2. upload the picture to the web. in the edit post mode, click on the html tab and use the following code:
http://www.quackit.com/html/codes/html_image_link_code.cfmxo
kittee