[котвася]

решил:

Код:

    protected void forword_Click(object sender, ImageClickEventArgs e)
    {
        string directory = Server.MapPath(@"foto_all/");
        DirectoryInfo dir = new DirectoryInfo(directory);
        FileInfo[] imageFiles = dir.GetFiles("*.jpg");
        ++i;
        if (i >= imageFiles.Length)
            i = 0;
        ImageMap1.ImageUrl = "foto_all/" + imageFiles.ElementAt(i);
        
    }