The photos may have been kept as a souvenir; a trophy of sorts. Add to this, that the girls' underwear had been "rolled off" of the remains, indicating sexual assault…He's where he needs to be, at least (if it is indeed this Eways character).
On another note, when perusing various news articles about the IDs, I noticed that very little was said about Martha or how her family is coping with it. It's all Sandy's family, in the photos and interviews. I am sensing a tiny bit of resentment from Sandy's family towards Martha, maybe because Martha was the older teen, and could have done a better job of keeping them both out trouble. Even though both girls were referred to as "wild" by the family. I have only to wonder what that even means, because these are teenage girls we're talking about, who aren't known for having the best critical thinking skills, and these were indeed the "flower power" years of free love, drugs and rock 'n roll. Did they smoke pot; were they promiscuous? Martha was evidently 17 years old - why did she marry Sandy's brother so young? Was she pregnant, and the families insisted on a marriage to "make things right," only to have the baby miscarried or stillborn? Maybe she was even the one to influence Sandy to come run away with her. She was tired of being tied down to hearth and home, at only 17… and we can only speculate that Sandy probably viewed her as a cool, older sister figure (I didn't hear any mention of other siblings in the news articles besides Hazel, or, any siblings older than she, leading me to conclude that she was the firstborn). After all, they were found with basically the same exact pair of sandals, as though Martha had seen them in a store window and wanted a pair, and so Sandy just had to have the same pair. It seems like these girls were just hanging out with the wrong crowd and were living in a time when there weren't a ton of resources set up for investigating the disappearance of a couple of teen runaways, which was all too ubiquitous. This really went under LE's nose for way too long, it's ridiculous in my opinion. That being said, my money's on Eways as being the killer.