I extract page images using this code:
for (int index = 0, size = doc.getNumberOfPages(); index < size; index++) {
    for(float zoom: new float[]{1f, 1.5f, 2f}) {
        IDocumentPage page = new DocumentPage();
        BufferedImage image = (BufferedImage) doc.getPageImage(index,
                GraphicsRenderingHints.SCREEN, Page.BOUNDARY_CROPBOX, 0f, zoom);
        try {
            ByteArrayOutputStream bos = new ByteArrayOutputStream();
            ImageIO.write(image, "png", bos);
            page.setNumber(index);
            page.setZoom(zoom);
            page.setByteData(bos.toByteArray());
        } catch (IOException e) {
            throw new IllegalArgumentException("Can't divided document to pictures", e);
        }
        saveImage(page);
    }
}

After migrating to IcePDF 5.1.0 image quality became poorer. See attached example and source pdf. Changing properties described here http://www.icesoft.org/wiki/display/PDF/System+Properties doesn't help at all.