It's easy.
In real life, to say tell somebody to change their body you say "change your body" but in Graal, you say "this.body = "body_nicholas.png" ;
I don't really recommend help from others, but ask when you need to.
What I recommend is to become a admin on a Dev. Server on PW and Press F9 or type /NPC and then press on the NPC's and examine them, add a new NPC and script it. You will remember it.
And trav, thank you. I haven't been able to check it out because my laptop screen is black but turns on eh..