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I am running Windows Vista, IIS 7 server. I submitted a URI for validation on the W3C MobileOK Checker. I received (repeatedly) the following error/warning: "Unexpected HTTP failure status code received" The "?Where" information says the error was generated by a request to retrieve an image, "Triggered by http://ddc3.pagekite.me/lwvvc-original/img/facebook-m.png." The link in the error description, above, is active and immediately retrieves the image when clicked. If I replace the image in the checked URI with an identical jpeg, MobileOK apparently does not have a problem retrieving the image--no error message is generated. Specifically, here is the html that generates the retrieval error: <img src="img/facebook-m.png" width="30" height="30" alt="Link to Facebook" /> Here is the changed html that does not generate an error: <img src="img/facebook-m.jpg" width="30" height="30" alt="Link to Facebook" /> Switching from png to jpeg changed my MobileOK score from 93 to 98.