Both of our image to gcode programs can be used for spindle engraving photos like lithophanes, Varied Intensity laser engraving or TTL (pulsing) B&W laser engraving images.
When TTL Pulsing the laser like for B&W Logos/Dithered images, you do not need to change your firmware because there are only two S command values (on/off) in the gcode file. It will still Dwell slightly, but there is no grayscale it can effect. Laser engraving with Varied Intensity (on-the-fly), you need to use the J-Tech LaserMode grbl 9g. The Variable S command values on each line will execute in real time without any dwelling.