A few thoughts:
1. Schools want the pictures to be something that people like -- not a source of anger/bitterness/envy/cynicism/frustration. So a polite (emphasis on polite) mention that it seems like it is not a wide variety of students in the photos, and maybe the school could mix it up on which classrooms get visited and which kids get photographed, could be feedback the school would find useful. Again, they're not putting up photos in the expectation it will increase parent dissatisfaction.
2. This is assuming that there is some section of the website with photos of the week or something like that. If it is just general website photos, they may have an outside photographer coming in twice a year so you won't see a lot of turnover of photos, and the photographer (as others mentioned), is going to go for photogenic kids/groups.
3. Even with #1, above, consider if you want to use up "capital" complaining about something that, while irritating to you, does not sound like it directly impacts your child's experience. There is a "boy who cried wolf" phenomenon -- certain parents become known as frequent complainers and that can dilute the impact of your communication. In other words, think about "saving your powder."
4. If you believe this is a manifestation of ingrained and corrupt favoritism, perhaps you should re-examine if it is healthy for you to have your child at a school that you distrust so much (for example, use of the word "sickening" seems very strong). If you truly hate the atmosphere and think terrible people are in charge, why keep your child there? Conversely, if you are generally happy with the school and think highly of the teachers in particular, perhaps try to be less angry/emotional about relatively minor issues like this and focus on whether big picture you like the education your child is getting.