This is the part in question:
Maybe as you just watched that you thought "whoa, did I just see Christopher Reeve?"
Look at the gif again. At the very beginning you can see that the lips and nose are thinner:
Here's the frame that looks most like Reeve:
The frame in question stands out even more when it's next to the frame right after it:
According to a redditor's comment, an editor on the film says no one has admitted to adding Reeve's face. But it's kind of hard to deny that you see something.
Maybe it's the lighting.
Or that the actors look a bit similar (after all, they are both Superman).
Whatever the answer is, it's nice to kind of see you again old friend!
h/t Uproxx