Since it's a server control, it's expecting the "onclick" attribute to contain a server-side method as its value. That's one problem.

It's been awhile, so this might not be 100% correct, but I think what you need to do is code the server-side ItemDataBound event. Within that method, you check the argument type, and if the control is an image button, then use the control's ".Attributes.Add()" method to code your onclick statement.