You are using asp:Image control and you want to display another image when user clicks on a button . Then Why you are using Java Script for this purpose.
In the same control itself you can load another image. In code behind you can try
protected void Button1_Click1(object sender, EventArgs e)
{
Image1.ImageUrl = "~/image/myImage.bmp";
}
here 'image' is the folder in the project root.
If you are using java script and hiding one control and the making other visible means , you have to load the both the image on the page load.
Hope this will help you :)