A generic error occurred in GDI+ at System.Drawing.Image.Save

When you are loading an imagefrom a Stream, You have to keep the stream open for the lifetime of the image, see this MSDN Image.FromStream.

I think the exception is caused because the memory stream gets closed even before the image gets disposed. You can change your code like this:

byte[] bitmapData = new byte[imageText.Length];
bitmapData = Convert.FromBase64String(imageText);

  using (var streamBitmap = new MemoryStream(bitmapData))
  {
      using (img = Image.FromStream(streamBitmap))
      { 
         img.Save(path);
      }
  }

Here are some links to threads discussing similar problems:

gdi+ error saving image from webpage

When drawing an image: System.Runtime.InteropServices.ExternalException: A generic error occurred in GDI