First, you can find all the icons used for a specific file type in the following folder: ~\App_Themes\Default\Images\FileIcons\ and the icons used for document types can be found here: ~\App_Themes\Default\Images\DocumentTypeIcons\

Inside the search results transformation you could use <%# GetSearchValue("classname")%>. This example code should return the code name of the given document type, like CMS.Blog for blog document, CMS.MenuItem for page (menu item) document, and so on. Then inside the src property of image control you can link to the file icon for the given document type

<img src="<%# "/some/url/" +GetSearchValue("classname").ToString() + ".gif" %>" /

Best Regards,
Edward Hillard