ThumbGeneratorAdapter picture resolution very low. Any idea?

Apr 13, 2013 at 4:14 PM
Following is my code, the uploaded file is high resolution, but the thumb images generated by thumb generator adapter has a very low quality, it works, but the quality is so low that it's not usable. Has anyone experienced 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="1200" />
thank you.