What rules are there for acceptable or unacceptable profile images for the C SE community?
Like with most offensive things, it can be hard to define strict rules, but you know it when you see it. Meta Stack Exchange has some general principles. If you see someone has a profile image that you think is offensive, write a custom flag on one of their posts to alert the mods to them. If they don't have any posts, then alert the mods in chat I guess.