Are you sure that you are providing a correct byte array? I just tried the following and it works without any problem:
using (MagickImage image = new MagickImage("logo:")) { using (MemoryStream stream = new MemoryStream()) { image.Format = MagickFormat.Png; image.Write(stream); stream.Position = 0; using (MagickImage output = new MagickImage(stream.ToArray())) { } } }