Hi try this Code, I am not sure ,hope it may work for you
Image fullSizeImg = null;
byte[] imageBytes = Convert.FromBase64String(base64String);
MemoryStream ms = new MemoryStream(imageBytes);
fullSizeImg = Image.FromStream(ms, true);
System.Drawing.Image.GetThumbnailImageAbort dummyCallBack = new System.Drawing.Image.GetThumbnailImageAbort(ThumbnailCallback);
System.Drawing.Image thumbNailImg = fullSizeImg.GetThumbnailImage(700, 800, dummyCallBack, IntPtr.Zero);
thumbNailImg.Save(ImagePath, System.Drawing.Imaging.ImageFormat.Png);
fullSizeImg.Dispose();
thumbNailImg.Dispose();