Explanation:

string[] images = new string[] { "/images/Backcolor.gif", "/images/Bold.gif", "/images/cut.gif" }; 

System.Random r = new Random();
string returningImage=images[r.Next(0,images.Length)]; 

string path = Request.ApplicationPath + GetRendomImage();

Image1.ImageUrl = path;
Image1.AlternateText = path;

protected void Page_Load(object sender, EventArgs e)
{
string path = Request.ApplicationPath + GetRendomImage();
Image1.ImageUrl = path;
Image1.AlternateText = path;
}
protected string GetRendomImage()
{
System.Random r = new Random();
string[] images = new string[] { "/images/Backcolor.gif", "/images/Bold.gif", "/images/cut.gif" };
return images[r.Next(0,images.Length)];
}