Che sia la compressione JPEG troppo alta? Modifica il parametro necessario per settare questo valore.

codice:
    long nQuality = 100;

    ImageCodecInfo[] icf = ImageCodecInfo.GetImageEncoders();
    EncoderParameters encps = new EncoderParameters(2);
    EncoderParameter ep1 = new EncoderParameter( System.Drawing.Imaging.Encoder.Quality, nQuality );
    EncoderParameter ep2 = new EncoderParameter( System.Drawing.Imaging.Encoder.Compression, 90 );

    encps.Param[0] = ep1;
    encps.Param[1] = ep2;
   
   thumbNailImg.Save ( Request.PhysicalApplicationPath + imageUrlThumb +   MyString , icf[1], encps);
   thumbNailImg.Dispose();