Pixelate image with average cell color
Jerry Coffin suggested that I should simply scale the image down to a smaller size, and then back up to the original size. (Thanks!)

Using Emgu CV, I was able to shorten my function to a single return statement.

private Image&lt;Bgr, byte&gt; PixelateImage(Image&lt;Bgr, byte&gt; image)
{
    return image.Resize((int)(image.Width * .25), (int)(image.Height * .25), Inter.Area)
        .Resize(image.Width, image.Height, Inter.Nearest);
}