You can use the alt attribute of the images and the AutoTooltipify feature of RadToolTipManager:
You can also try the approach from this demo: http://demos.telerik.com/aspnet-ajax/tooltip/examples/databasetooltipswithoutlod/defaultcs.aspx
. Once you generate the Ids of the images you can add them as targets for the manager and add a specific HTML string for them. Note you may need to escape it to prevent it from breaking the page.
All the best,
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed