As far as I can tell, StackOverflow does nothing about this.
From the Stack Exchange Network Content Policy:
You can't flag a user, but you can (as ChrisF already mentioned) flag one of their posts and explain the issue. A moderator will then deal with it.
Stack Exchange can't possibly do a skin tone analysis on every image that people post in their profile. Much like the rest of the features on the site, we trust you to behave professionally and in accordance with the rules.
If you fail to do so, we'll let you know. That's why we're here.
If you find something inappropriate in a profile, flag one of the profile owner's questions or answers and let us know what's wrong. We'll handle it :)