Open in a new window

Hello, In my web application i had placed a grid view with image button in the template field of the gridview. I want to display advertisment using this Imge button. I had bind the picture to the image button using the command Eval("fieldname") in the edit templates option. an i had set the Postback url also in the same option. So that when i click on the image. I am reirecting to the url which i given. But i want to open that advertisment in a new window. Pleas help me ...