Inside RadGrid_RowDataBound client side function you can access the column - columns[i].get_element().UniqueName where UniqueName is "Tumbnail". Then you need to implement a client side method to replace the dynamic url.
All the best,
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
for a walk through all the exciting stuff we ship with the new release!