Web Forms :: Set The Image Per The Session A Users Visits The Site Instead Of Each Page?

I have a background set of images that is called randomly.It changes each time a user opens a new page and refreshes the same page.Not 100% of what I want.

Ideally I would like a random image to be shown each time a user comes to the site but not on every page.

Is there a way to set the image per the session a users visits the site instead of each page?

Current code

SqlConnection connection = null;
connection = new SqlConnection(ConfigurationManager.ConnectionStrings["PSI_DB_1ConnectionString"].ConnectionString.ToString());
connection.Open();
int id = Convert.ToInt32(Request.QueryString["id"]);
string SQLquery ="";
if (id == 0)
{
SQLquery = "Select Top 1 ImageData,ImageType from WebBGImages ORDER BY NEWID()";
}
else
{
SQLquery = string.Concat("Select ImageData,ImageType from WebBGImages where ImageID=",id);
}
SqlCommand Cmd = new SqlCommand(SQLquery, connection);
SqlDataReader Dr = Cmd.ExecuteReader();
if (Dr.HasRows)
{
while (Dr.Read())
{
Byte[] bytes = ((Byte[])Dr["ImageData"]);
Response.Buffer = true;
Response.Charset = "";
Response.ContentType = Dr["ImageType"].ToString();
Response.BinaryWrite(bytes);
Response.End();
Response.Cache.SetCacheability(HttpCacheability.NoCache);
Response.BinaryWrite(bytes);
}
}
Response.Flush();
Response.End();
Cmd.Dispose();
connection.Close();
connection.Dispose();