As an artist, I wonder: is there any other profession where people expect naturally that you'll do stuff for them for free? Always with the mention: It's PR for you and I'll tell people about your work!
Yeah thanks, unfortunately the grocery store does not take nice words from "friends" as a payment, neither do the phone companies, tax offices or any other.
Being an artist is a job, I provide I service and services are to be paid for! Would you ever tell an architect to build you a house with the words: "the house will be visible to everybody, it's PR for you, I'll tell my friends about it, you owe me!" Eh?
Advice to new, little experienced artists out there: you deserve to be paid for your work, be it a painting, a sculpture, a photosession or any other creative work! Everything else is abusing your kindness, and lessens the value of your work!