Popular mods, house rules, RPGs, modules have been disturbed freely on the net since the 1990s.
The law is worded to never be on your side, so it is scary. But it is unlikely a photographer/artist/musician/publisher will send you a cease and desist for such a small niche product making no profit. At best, they may want citation credit.
If you want to perform your due diligence to sleep well at night, track down the copyright holders and respect their wishes. Otherwise only use public domain resources.