Image control in asp.net c#

Friends !

               I have image1 control on the webform1.aspx. I give it imageUrl as defaultImage.jpg or with any extension. when I click on the button1 control then I want to change the imageUrl permanently.

               For the time being when I click on the button Image changed but when I cross my browser and run my application again then defaultImage show again and I want to display the second Image on the webform. I need help. Thanks a lot.