ThumbGeneratorAdapter picture very low quality, any idea?

Apr 13, 2013 at 4:18 PM
following is my code. the FileSaverAdapter uploads the original file in high quality, not a problem.
the ThumbGeneratorAdapter generates the thumb image file in a very low resolution/quality, making the image barely recognizable. Anybody has experienced the similar issue?
    <fjx:FileUploader ID="FileUploader1" ImagesPath="~/upload" MaxFileSize="1MB" MaxFileQueueSize="4MB" runat="server">
            <fjx:FileSaverAdapter FolderName="upload" Runat="server" />
            <fjx:ThumbGeneratorAdapter Runat="server" 
                Extensions="jpg;jpeg" FolderName="upload"
                Prefix="l-" MaximumWidth="640" MaximumHeight="640" />