If you define your items statically you can just set the ToolTipService.ToolTip attached property directly to the container (if you use directly images you can replace them with RadCoverFlowPerspectiveItems and set the property on them).
If your items come as ItemsSource you could set this attached property through the ItemContainerStyle property of the CoverFlow like this:
| <Style TargetType="navigation:RadCoverFlowPerspectiveItem">
| <Setter Property="ToolTipService.ToolTip" Value="ToolTip" />
doesn't support binding in the style like WPF
and because of that you can use this code only for a static ToolTip
If you need to bind your tooltip to some data you need to do something more complex. I would recommend you to implement attached behavior that will create the binding.
Hope this information is helpful.
the Telerik team