There are a couple of things that you could try.  Try to scan at the same Resolution that you are intending to engrave at.  If you can't do that, then in Photopaint, after you resize an image, go to Image | "Resample..." and change the DPI to whatever you will be engraving at.  This creates a 1:1 ratio between the pixels you have and the pixels that the LASER wants.

Then in Photopaint, before you bring it into PhotoGrav, after converting to 8-bit grayscale go under the Image menu and chose "Adjust brightness, contrast, intensity"  I usually like to make the image a little brighter and turn the contrast up quite a bit.  The next thing is to go under the Effects menu and under Sharpen, Choose "Sharpen"  This can make your image look a lot crisper.  While you are doing this make sure to be looking at the image at 100% zoom so that you can see the true effect sharpen has on your image.

Now you can either just engrave the image, or you can import it into
Photograv and continue with your process.  I do not use Photograv all that much so I cannot really offer you any suggestions as how to use that.  You may also be using too much Power, it is a very common thing that I see with photographs.  Lighter often turns out better.