Handle GridView RowDataBound Event, and in that write javascript for e.Row.Attributes for onmouseover, in the javascript just show the image in a modalpopup...to get started - http://www.codeproject.com/Articles/18876/Using-JavaScript-To-Select-GridView-Rows 

http://www.iamraghuveer.com/2012/04/display-gridview-row-details-using.html

Thanks,