For some users in the Top Askers section on the Hot QuestionsHot Questions page are wrong profile pictures displayed. Especially for the users that are on top of that list. May be it is a caching problem or that the users have changed their profile picture recently.
As an example use User Simar.
Picture as Top Asker (Links to his profile on Mathematics)
Picture on Profile (Mathematics)
The "birdy picture" is used in the profiles on Mathematics and StackExchangeStackExchange. The profiles on Chemistry and Physics use the identicon.
It seems that the wrong profile picture is used as Top Asker. At least not the picture of the profile that is linked to.