Glass is probably the most difficult material on which to engrave a photo so be patient and practice (cheap window glass is a good & inexpensive practice material).
First, select the “Ornament Glass Standard” from the material selection dialog. This material option has all the parameters set for glass, except recommended speed and power which will be discuss later.
Generally, for engraving on glass ornaments, it is recommended to make the photo rather contrasty and try to not have any very bright areas. Typically one should set the max gray shade (using Corel PhotoPaint or Adobe Photoshop) to about 192 instead of 255.
In general, you will have to go into "Interactive Mode" (after opening the image), move the white triangle all the way to the right (this prevents PhotoGrav from spreading all the gray shades over the entire range - in this case it prevents it from resetting all the lighter gray shades to whites). Then click "Final Process" to process the image.
Two other points: (1) if you remove the background from the photo for engraving on glass then make the background all black (before processing in PhotoGrav). This causes PhotoGrav to NOT engrave that black area which generally looks better than an engraved white background and (2) When you go into "Interactive Mode", you might want to make the "Noise Gain" parameter about 10-15%. This has the effect of introducing some texture in the whiter grayshade areas, and than, after engraving, makes them appear a little frostier and not so "shattered" looking.
For a 50 watt machine, good results can be achieved on glass with a Speed (S) = 80%, Power (P) = 80%, and engrave at 300 dpi while usually resampling the image to 150 dpi. Scale these values appropriately for your machine. However, since lasers vary a lot from their rated power, you will have to experiment to get the best S&P settings for your machine.