As a general rule I can take or leave pictures. I only really used them for GZ and Necro since I was sort of experimenting with using them. I probably won't bother doing it again, mainly because it's really damn difficult to find that exact picture that you're looking for.
I think I probably searched for every post apocalyptic pic on the net when I was writing GZ and still didn't necessarily find what I needed. In some cases I used something else instead (Hey zombie pics get the point across that mutants ate you just as well). I was using a lot of movie and video game pics, comics as well. Most of it from the 80s since there was a shitload of Mad Max rip offs and quite few video games with post apocalyptic themes (Used the original Fallouts quite a bit obviously). Of course if I was making Ground Zero now I'd probably find pictures a lot easier given stuff like Fallout 3, Borderlands, Book of Eli, The Road and a bunch of similar stuff has come out since then!
Necro was sort of easier. It was a lot shorter for one thing and there's LOTS of fantasy pics out there. My main thing was just finding the right kind of cloaked faceless figure to represent the Necromancer in certain parts. Plus I was adding text to certain pics. The pic on the front with the Necromancer and his queen before you actually click on the story was shopped so that the cloaked figure and that girl are together. I'm still sort of surprised how good I managed to make it look considering I was using MS Paint and my art skills are crap. (Hence why I wasn't just drawing this stuff to begin with)
I also felt it was important to add a pic to every page I created too, so that it wouldn't look inconsistant. I almost added pics to Death Song when I was doing that one, but ultimately I was having a harder time finding the right ones for certain pages so I decided it would be easier to forget the pics and just leave the story pictureless.