The main need is not about Glowforge but a good image editor. I use Gimp as the only cost is the not minor effort to learn it. ((Those you buy have that effort also) There are many possible results very dependent on the look you have and the look you want.
You will need a sharp, high contrast b&w image and use the dots and not depth cut unless you have the skills to create an elevation style map that looks very different from a photo.
When you have a great photo, the Glowforge part is a snap.
I cheat, and I am sure its a cheat that you can do a LOT easier and cleaner then my cheat.
that said, I take a photo, and delete the background (in photoshop)
then I use an “outline” tool, to create a 2 pixel outline of the photo, THEN I erase the picture,
leaving nothing but the outline. I then save that outline as a .pst, import it into illustrator
and then auto draw that outline (clean up any hash) save it as a .svg - and now as a vector I can set the line to be cut.
PS: if you are a premium member, there is a ‘outline’ tool available in the GFUI
Gimp is no all that different, but there is a lot more in dealing with the kind of shading that makes an image strongly recognizable as that person and that is a much bigger challenge than a proper outline.