Hi dilipsharm,
Assign runat="server" property to the img tag so that you can access in code behind and set the url and remove attribute like click event.
Check the example.
HTML
<img runat="server" id='img07' alt='Unbooked' onclick="seat('07')" src='images/chair/G_chair.png'
style='cursor: pointer;' />
C#
// Set Scr property.
img07.Src = "";
// RemovedControl onclick event.
img07.Attributes.Remove("onclick");