There's a conspiracy theory going around that he was killed by The Man, who is resorting to tricks like CGI to pretend he's still alive.
I'm not sure what the motivation for killing him and continuing to pretend that he's alive would be, but then again, I don't understand most conspiracy theories.
Oh, but there are easier ways to do that. Offer to let him avoid being extradited to Sweden in exchange for not reporting on certain things, for instance.
Do you really think the Ecuadorean embassy, which had to install a shower so they could house him, is able to resist political pressure from basically all of NATO? Snowden had to flee to Russia, not even a Russian embassy, and he's not even leaking anything new. (I'm not necessarily claiming that anyone is actively manipulating WikiLeaks, just that there are a million easier ways to do it than secretly killing him and making a CGI double - and secretly killing him would require Ecuador's cooperation, anyway.)
That rumor started in November 2016, or a bit earlier, when Assange went dark for awhile, right after tweeting about a large police presence outside the embassy.
I don't think it's a very popular conspiracy theory that Obama would have killed him. Just a few people who remember SECSTATE Clinton talking about droning him, and that has given them fuel for this speculation.
I can't deny there's a resemblance... but he sure looks different now... older. A lot older.
Edit: Here's a recent photo of him [1]. The difference might be the lack of a beard...
1. http://www.abc.net.au/news/image/7145700-3x2-940x627.jpg