SEO Canonical Issue resolution on iis

Question

The page with URL "http://www.site.org/images/join_forum.gif" can also be accessed by using URL "https://www.site.org/images/join_forum.gif".Search engines identify unique pages by using URLs. When a single page can be accessed by using any one of multiple URLs, a search engine assumes that there are multiple unique pages. Use a single URL to reference a page to prevent dilution of page relevance. You can prevent dilution by following a standard URL format.

this is the error i am getting