If you ever want to engrave a greyscale image using vector engraving (using one really long continuous curve) rather than using a raster engrave, try out Image2Hilbert

Here’s a video of someone actually engraving an example.

It turns an image into a sort of modified Hilbert Curve (a well known space-filling fractal). It even calculates the length of the curve, so you can estimate how long it will take to engrave given a particular speed setting.