How to get the path to the internal memory of the smartphone and the DCIM folder, then to save the photo there?
string directory = System.IO.Path.Combine(Android.OS.Environment.ExternalStorageDirectory.AbsolutePath, Android.OS.Environment.DirectoryDownloads);
FileStream file = new FileStream(System.IO.Path.Combine(directory, "newProdict_" + product.Width + "x" + product.Height + ".png"), FileMode.OpenOrCreate);
You can get the DCIM folder like that:
For writing files you can use the nuget package PCLStorage so you already have everything in a cross platform manner: