Some example snippets from around the web related to low-level XNA Texture2D manipulation.

Texture2DStuff.cs

//To convert a Texture2D to an Image :

public static System.Drawing.Image Texture2Image(Texture2D texture)
{
  if (texture == null)
  {
      return null;
  }

  if (texture.IsDisposed)
  {
      return null;
  }

  //Memory stream to store the bitmap data.
  MemoryStream ms = new MemoryStream();

  //Save the texture to the stream.
  texture.SaveAsPng(ms, texture.Width, texture.Height);

  //Seek the beginning of the stream.
  ms.Seek(0, SeekOrigin.Begin);

  //Create an image from a stream.
  System.Drawing.Image bmp2 = System.Drawing.Bitmap.FromStream(ms);

  //Close the stream, we nolonger need it.
  ms.Close();
  ms = null;
  return bmp2;
}

//To convert an Image to a Texture2D:

public static void Image2Texture(System.Drawing.Image image,
                                 GraphicsDevice graphics,
                                 ref Texture2D texture)
{
  if (image == null)
  {
      return;
  }

  if (texture == null || texture.IsDisposed ||
      texture.Width != image.Width ||
      texture.Height != image.Height ||
      texture.Format != SurfaceFormat.Color)
  {
   if (texture != null && !texture.IsDisposed)
   {
          texture.Dispose();
   }

   texture = new Texture2D(graphics,image.Width,image.Height,false,SurfaceFormat.Color);
  }
  else
  {
      for (int i = 0; i < 16; i++)
      {
          if (graphics.Textures[i] == texture)
          {
              graphics.Textures[i] = null;
              break;
          }
      }
  }

  //Memory stream to store the bitmap data.
  MemoryStream ms = new MemoryStream();

  //Save to that memory stream.
  image.Save(ms, System.Drawing.Imaging.ImageFormat.Png);

  //Go to the beginning of the memory stream.
  ms.Seek(0, SeekOrigin.Begin);

  //Fill the texture.
  texture = Texture2D.FromStream(graphics, ms, image.Width, image.Height, false);

  //Close the stream.
  ms.Close();
  ms = null;
}

private void RemovePlanetChunk(Vector2 RemovePosition)
{
    // Create a render target, which we will draw to instead of the screen
    RenderTarget2D target = new RenderTarget2D(graphics.GraphicsDevice, _Planet.Width, _Planet.Height);

    // set the RenderTarget2D as the target for all future Draw calls untill we say otherwise
    graphics.GraphicsDevice.SetRenderTarget(target);

    // start our batch as usual..
    _SpriteBatch.Begin(SpriteSortMode.Immediate, BlendState.AlphaBlend, SamplerState.PointClamp, null, null);

        // start with a transparent canvas
        graphics.GraphicsDevice.Clear(Color.Transparent);

        // add in the previously drawn dots from the current alpha map.
        _SpriteBatch.Draw(_AlphaMask, new Vector2(0,0), null, Color.White, 0f, Vector2.Zero, 1f, SpriteEffects.None, 1f);

        // add a new dot to the map.
        _SpriteBatch.Draw(_CuttingDot,
                RemovePosition,
                null,
                Color.White,
                0f,
                new Vector2(_CuttingDot.Width / 2f, _CuttingDot.Height / 2f),
                _HoleSize,
                SpriteEffects.None,
                1f);

    // end the draw call
    _SpriteBatch.End();

    // start drawing to the screen again
    graphics.GraphicsDevice.SetRenderTarget(null);

    // set our Texture2D Alpha Mask to equal the current render target (the new mask).
    // RenderTarget2D can be cast to a Texture2D without a problem
    _AlphaMask = target;
}