example.cs

#r "System.Drawing.dll"

using System.IO;
using System.Drawing;
using System.Net;

var client = new WebClient();
var bytes = client.DownloadData("http://www.hdwallpapersplus.com/wp-content/uploads/2013/05/Funny-Cat-Pictures-cats-935656_500_375.jpg");
var stream = new MemoryStream(bytes);
var image = Image.FromStream(stream);
Console.WriteLine("size: {0}x{1}", image.Height, image.Width);