You should not write an jpeg and then open it to modify it, you will lose image information. With the following example I am able to create the page with a white background:
MagickReadSettings settings = new MagickReadSettings(); settings.Density = new MagickGeometry(144, 144); settings.ColorSpace = ColorSpace.sRGB; using (MagickImageCollection images = new MagickImageCollection()) { images.Read("29_6a Farmout Flyer April 2012.pdf", settings); MagickImage coverPage = images.First(); coverPage.Resize(0, 500); coverPage.ColorAlpha(new MagickColor("#fff")); coverPage.Quality = 80; coverPage.Write("PJPEG:29_6a Farmout Flyer April 2012.jpg"); }