Here is quick sample code that fails for me:
https://dl.dropboxusercontent.com/u/9762525/thumb.bmp
MagickImage magickImage = new MagickImage(@"c:\thumb.bmp");
magickImage.Modulate(new Percentage(90), new Percentage(100), new Percentage(100));
magickImage.Write(@"c:\thumb-mod.bmp");
here is link to file used in this example:https://dl.dropboxusercontent.com/u/9762525/thumb.bmp