The basic rule most of us follow is that if there is no profit being made, or any profit goes 100% to charity, and we are talking a limited number you are probably safe from the lawyers. LFL is not going to bother somebody who makes a 100 or so shirts for charity. 20,000 shirts being sold in a local store, thats a no no. Our FF chapter had T-shirts made up by a local shop. He sold them to us for his cost, which was around $8 per, we sell them for $10 and the $2 goes to the charity. So as long as what you are doing is reasonable in the use of the images etc, you should be OK.