Основываясь на ответе @peters, я использовал это:
string bitmapString = null;
using (MemoryStream memoryStream = new MemoryStream())
{
image.Save(memoryStream, ImageFormat.Png);
byte[] bitmapBytes = memoryStream.GetBuffer();
bitmapString = Convert.ToBase64String(bitmapBytes, Base64FormattingOptions.InsertLineBreaks);
}
и
Image img = null;
byte[] bitmapBytes = Convert.FromBase64String(pictureSourceString);
using (MemoryStream memoryStream = new MemoryStream(bitmapBytes))
{
img = Image.FromStream(memoryStream);
}