First, with PicSender set to 'Spindle' mode, manually jog your Z height to where you want it and click the blue 'Zero Z' button.
Then you must switch grbl to laser mode. To do this:
Click the yellow GRBL button and check the 'Use Auto Settings' check box on the Grbl set up screen.
Then, click click on one of the two laser mode options. For images this will be 'Laser Raster'. This will allow PicSender to set grbl to laser mode.
Click the 'Return' button to go back to the main screen.
Your gcode file will not use 'Z' commands for laser image engraving. It will use 'S' commands to set the power.
Set the 'S' power setting to 5 and click the 'S+' button when you are ready to turn on the laser to low power mode. Your laser should light up at this setting. If it does not, click the 'S+' button again to increment the power to 10. Your laser should be light up to low power then.
Jog to you desired X/Y zero position and set X and Y to 0 (zero) by clicking the X and Y blue Zero buttons.
You should now be all set to start engraving by sending a gcode file.
Make sure the laser engraving gcode file has been generated with no 'Z' commands in it.