后台：

foreach (DataListItem item in dlContent.Items)

{

    System.Web.UI.WebControls.Image img = (System.Web.UI.WebControls.Image)item.FindControl("userphoto");

    string fileName = Request.PhysicalApplicationPath + img.ImageUrl.Replace("/", @"\");

    System.Drawing.Bitmap objPic = new System.Drawing.Bitmap(fileName);

    int height = objPic.Height;

    int width = objPic.Width;

    if (height >= width)

        img.Attributes["height"] = "130";

    else

    {

        img.Attributes["width"] = "100";

    }

}

前台：

<asp:Image ID="userphoto" ImageUrl='<%# Eval("filename") %>' runat="server" CssClass="image"/>