'System.Web.UI.WebControls.Image' does not contain a

我试图从web服务获取图像，但遇到了一个错误
"System.Web.UI.WebControls.Image"不包含"FromStream"的定义我的代码。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;

namespace SmartPoster
{
    public partial class _Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

            // call web service here
            SmartPosterV1 a = new SmartPosterV1();
            MemoryStream mem = new MemoryStream(a.returnTestImage());
            this.pictureBox1.Image = Image.FromStream(mem);                
        }
    }
}