CreateImageFromStream

using System;
using System.Net;
using System.IO;
using System.Windows.Forms;
using System.Drawing;

class ストリームからイメージを作成する
{
    [STAThread]
    static void Main(string[] args)
    {
        Application.Run(new MyForm());
    }
}

class MyForm : Form
{
    public MyForm()
    {
        String uri = @"http://livedoor.blogimg.jp/borocso/imgs/3/6/36b40029.jpg";

        PictureBox pic = new PictureBox();
        this.Controls.Add(pic);
        this.Size = new Size(600, 400);
        pic.PointToClient(new Point(0, 0));
        pic.ClientSize = this.ClientSize;

        Image image = null;
        WebClient wc = new WebClient();
        using(Stream stream = wc.OpenRead(uri))
        {
            if(wc.ResponseHeaders["Content-Type"].StartsWith("image/"))
            {
                image = Image.FromStream(stream);
                pic.Image = image;
            }
        }
    }
}