A few years ago they experimented with using stock photography for some of the pix, including the cover. There was one with a good looking blonde about unfaithful marriage mates. Then they decided since they didn't have control over the models (ie, she could wind up in a porn movie or something) that they would keep doing everything in house.
My favorite pictures are when they try to portray some seedy side of life, like gang activity. The witness models they use end up looking like rejects from the Village People. I doubt they would make it in the Crips or Bloods.