В GDI+ произошла общая ошибка: в моем коде появляется это исключение, когда я пытаюсь сохранить оптимизированное изображение на сервере

Question

Вот мой код, который я использую для перерисовки и сохранения изображения.

Bitmap bitmap = new Bitmap(HttpContext.Current.Server.MapPath(
    filePath + Path.GetFileName(fileName)));

int newWidth = 100;
int newHeight = 100;

int iwidth = bitmap.Width;
int iheight = bitmap.Height;

if (iwidth <= 100)
    newWidth = iwidth;

if (iheight <= 100)
    newHeight = iheight;

bitmap.Dispose();

// ONCE WE GOT ALL THE INFORMATION, WE'll NOW PROCESS IT.

// CREATE AN IMAGE OBJECT USING ORIGINAL WIDTH AND HEIGHT.
// ALSO DEFINE A PIXEL FORMAT (FOR RICH RGB COLOR).

System.Drawing.Image objOptImage = new System.Drawing.Bitmap(newWidth, newHeight, 
    System.Drawing.Imaging.PixelFormat.Format16bppRgb555);

// GET THE ORIGINAL IMAGE.
using (System.Drawing.Image objImg =
    System.Drawing.Image.FromFile(HttpContext.Current.Server.MapPath(filePath + fileName)))
{

    // RE-DRAW THE IMAGE USING THE NEWLY OBTAINED PIXEL FORMAT.
    using (System.Drawing.Graphics oGraphic = System.Drawing.Graphics.FromImage(objOptImage))
    {
        var _1 = oGraphic;
        System.Drawing.Rectangle oRectangle = new System.Drawing.Rectangle(0, 0, newWidth, newHeight);

        _1.DrawImage(objImg, oRectangle);
    }

// On the below lines of code i am getting error while i try to execute 
// this code after uploading image. If image size is around 10 kb it will 
// not throw any error but if it exceeds then 10 kb then it will.          

    //SAVE THE OPTIMIZED IMAGE
    objOptImage.Save(HttpContext.Current.Server.MapPath(filePath + "TempSubImages/" + fileName),
        System.Drawing.Imaging.ImageFormat.Png);**

    objImg.Dispose();
}

objOptImage.Dispose();

// FINALLY SHOW THE OPTIMIZED IMAGE DETAILS WITH SIZE.
Bitmap bitmap_Opt = new Bitmap(HttpContext.Current.Server.MapPath(filePath + "TempSubImages/" + Path.GetFileName(fileName)));
byte[] bytes = System.IO.File.ReadAllBytes(HttpContext.Current.Server.MapPath(filePath + "TempSubImages/" + Path.GetFileName(fileName)));

UI.BasePage.TemporaryFile = bytes;
String[] ArrContentType = new String[] { postedFile.ContentType, fileName };
UI.BasePage.TemporaryStrings = ArrContentType;

int iwidth_Opt = bitmap_Opt.Width;
int iheight_Opt = bitmap_Opt.Height;
bitmap_Opt.Dispose();