I've been engraving photos for quite a few years now and haven't had much luck engraving 8 bit images. Unfortunately you'll most likely need to look into one of the 3D renedering applications to pull this off successfully. I only know of one that works pretty well and that's EngraveLab. It's a bit pricely though; however, a sales rep will work with you if you're already using CorelDraw. I recall somewhere around the $500-$600 price range a while back. We primarily use PhotoGrav as well as our own in house developed photo engraving application.
Best of luck to you and happy engraving!