IIS Search Engine Optimization and Canonical Url with Dynamic Image

Question

I get the following error below for my dynamic image generator. It can have any kind of width or height but SEO is complaining about it because it needs a canonical URL. How can I do this with a dynamic image that isnt a page?

The page with URL "http://localhost/assets/images/99?w=125" can also be accessed by using URL "http://localhost/assets/images/99?w=100".
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.

Answer 1

The canonical URL can be specified in a HTTP header, too. See RFC 6596, section 4:

or alternatively, in the HTTP header field as specified in Section 5 of [RFC5988]:

Link: <http://www.example.com/page.php?item=purse>; rel="canonical"