Outlook 2013 supports the inclusion of JPG, GIF, PNG and BMP image files directly in a signature. To add a PDF image, you must include it as a Web link.

PowerPoint 2007 saves presentations as Web pages, converting files to HTML with a single step. PowerPoint 2010 disabled this function, but it saves presentations as images which you can embed ...

Now if this was the HTML page itself, it would be pretty easy to send an HTTP header to clue in the browser (force expiry IIRC), but the browser will ask for the image in a separate connection ...