Change imageUrl of ImageButton without PostBack

I have an imageButton and I need to change the imageurl when clicking on it

It works fine using the button’s onclick event and C#:

if (Image.ImageUrl == “images/image1.gif”)
{
Image.ImageUrl = “images/image2.gif”;
}
else
{
Image.ImageUrl = “images/image1.gif”;
}

But it makes me postback. I need to change it without postback.

Thank you in advance