I don't see where you have attempted to add an image, but this would be possible only inside a series tooltip, because this is the only place in the chart that renders HTML, the rest is SVG/VML.
Take a look at these articles to see how to add HTML to your tooltips and bind it to database data, if needed (e.g., for the src attribute):
http://www.telerik.com/help/aspnet-ajax/htmlchart-client-templates-for-tooltips-and-labels.html