If there's really G-code in your image file, Optical Character Recognition should be able to recover it. Check it carefully for errors first, though - a missed decimal point or bad number could do a lot of damage.
If you're really talking about converting a 2D image to a heightfield mesh and generating code to carve it, DeskProto is my favorite tool for that. We sell it for the same price in number of dollars that it's priced in Euros (although that may change if the dollar keeps falling).