Either I'm not understanding what your trying to do, or your misunderstanding what I'm trying to explain how to do it.
The outer white areas in the image is not going to burn with the laser anyway, so why would you want it to travel in that area and take longer to finish?
GRBL PWM will generate the exact same gcode as Analog/PWM will if the settings are the same. The reason I suggested using the Analog/PWM profile because it has more features then GRBL PWM has including the Feed Rate Change. The FRC will add a varible feedrate to each line of gcode to slow in higher laser power darker shade areas and it speeds up the lower laser power lighter shade areas. Those settings on the forum such as the feedrate, Min 3rd Axis Depth and the Feed Rate Change percentage may need adjusted for the type of wood your engraving on. As I stated, there recommended starting place settings.
You only have PicLaser Lite which does not have the image editor like PicEngrave has. The Pixel Resolution setting is for the axis step over in the gcode so the laser's burn lines lay down next to each other without any gaps between them or overlapping. If you use any other Pixel Resolution setting than .254mm, you will have to resize your image to compensate for it, or it will not engrave the size you expect.
The PicEngrave image editor has an Auto selection that you input the Pixel Resolution value, then you adjust the Pixel Width or Height for resizing.