The following code is not particular clean or sexy but you can basically do the same thing avaiable in XNA. This may change a bit for cross platform projects because of portable libs, but it would be easy to change how you load the stream.
Texture2D localImage = null;
Texture2D fromWeb = null;
using (var filestream = new FileStream(@"tests.png", FileMode.Open))
{
localImage = Texture2D.FromStream(GraphicsDevice, filestream);
}
using (var client = new System.Net.WebClient())
{
var result =
client.DownloadData("http://opengameart.org/sites/default/files/Heroes_01preview.png");
using (var stream = new MemoryStream(result))
fromWeb = Texture2D.FromStream(GraphicsDevice, stream);
}
spriteBatch.Begin();
if (localImage != null)
spriteBatch.Draw(localImage, Vector2.Zero, Color.White);
if (fromWeb != null)
spriteBatch.Draw(fromWeb,
new Vector2(localImage != null ? localImage.Width : 0, localImage != null ? localImage.Height : 0),
Color.White);
spriteBatch.End();