string url_da_passare=String.Format("tuohandler.axd?id={0}",var_ID_recuperata_precedentemente);
TuoImageControl.ImageUrl=url_da_passare;

public class tuohandler:IHttpHandler{
public void ProcessRequest(HttpContext ctx){
int id=-1;
bool result=Int32.TryParse(ctx.Request.QueryString["id"], out id);
if(!result)
ctx.Response.End();
string connessione="........";
string command="select ...................... where IDfoto=@id";
byte[] img=null;
SqlConnection conn=new SqlConnection(connessione);
using(conn){
SqlCommand cmd=new SqlCommand(command,conn);
cmd.Parameters.AddWithValue("@id",id);
conn.Open();
img=(byte[])cmd.ExecuteScalar();
conn.Close();
}
if(img!=null){
 ctx.Response.ContentType="image/jpeg";// ad esempio
 ctx.Response.BinaryWrite(img);
}
}
}
public bool IsReusable{
get{return true;}
}
}