Because I love a challenge, I've been sitting here trying to think of situations in which it would be rude to post food pics. I came up with two:
1) If your picture includes a note saying, "Mine is way better than *insert name of Facebook friend - extra points for tagging*."
2) If you posted something that one of your Facebook friends is allergic to with a note saying, "Yummy, I feel sorry for anyone who can't eat this!"
But the comments are rude not the photos, and I really had to stretch to come up with them.