How to set Image Quality value when converting a PDF to JPG?

You can do this with the following example code in Java.

ConvertPagesToImages convert = new ConvertPagesToImages("inputFileName");
final JpegEncoderOptions options = new JpegEncoderOptions();
if (convert.openPDFFile()) {
    for (int page = 1; page <= convert.getPageCount(); page++) {
        BufferedImage bi = convert.getPageAsImage(page);
        //write out
        JDeli.write(bi, options, new File("outputFileName" + page + ".jpg")); 

You can find out more about the PDF to Image conversion here.