This looks like another fishbowl...not sure who to contact.
There are two CSS rules in https://rs.wikimedia.org/wiki/%D0%9C%D0%B5%D0%B4%D0%B8%D1%98%D0%B0%D0%B2%D0%B8%D0%BA%D0%B8:Common.css that are loading Twitter images from postimg.org. I suspect something in https://commons.wikimedia.org/wiki/Category:Twitter_t_logos could be used instead. But given that the postimg.org site no longer exists, removing them without an immediate replacement should be totally safe.