Image button onclick event is not working in user control

You don't have one image button in your project, you have many, all created by the datalist, not by you.

You will need to handle the datalist OnItemCommand method to be able to respond to the click event of all of them. I note that there are two pages of articles that reference OnItemCommand ( a method used by all of MSFT's databound controls, and one well worth understanding) right here on CP