As has been posted about already in this very forum, I'm intending to make a worldbook based on my game (see my sig). This would be printed and sold, rather than free on the net.
Obviously you should just contact the artists involved for this new use of their work.
However, if you don't care about the money, you could sell the worldbook for the cost of printing, so that the worldbook itself - its content - is free.
The problem would be if you wanted to have it available as a pdf, too. If it's available as a free pdf, almost no-one will get the printed version.
But if it's a free pdf everyone will download it but almost no-one will read it, and even less play it. If you want people to read and play it you have to charge for it - however token a sum.
You have to decide what your goals are in writing it, and shape your approach to that. When writing
d4-d4, my goals were,
- I had something to say, a game approach I thought was a good one
- to boost my ego by having what I'd written discussed
- and more importantly, played
- least importantly, make a few hundred bucks so I could buy more rpg stuff online
With those goals, even without the last one, I decided that charging for it was the thing to do.
Had I had art from people who'd given it on the understanding it'd be a free product and then changed my mind, I'd just tell them so and let them decide.