Well I think @andrew0 has suggested his site in one of the comments. Alternatively we also have @blueorgy's SteemImg - which is what I always use and has some quite advanced features compared to other hosting sites. I don't think it has to be associated with a single host but it might make it easier to find things if there is no way or intention to incorporate any of these features on the Steemit side of things. If you think about it the Steemit blockchain itself can act as a source of aggregation to an extent by storing all the links.