Go to your hub and click on edit. Once you're in edit mode you can add an image capsule. This will automatically be placed at the bottom of all your other capsules. Use the arrows at the top of the capsule to put the image where you want in the hub. Or if you want to draag and drop the capsules in the reorder box on the right.
Here's the link to the full instructions on the help pages: http://hubpages.com/faq/#editcapsule