I just added a simple unit test to see if there was a problem with the AppendHorizontally but that seems to work correctly. Are you using the collection after it has been Disposed? For example this:
Using images AsNew MagickImageCollection() images.Read(sPlantillaRellena, settings) images.Dispose() ' You also don't need to do this because of the using statement...Dim horizontal As MagickImage = images.AppendHorizontally() horizontal.Write(sRutaImagen) EndUsing