Another feature of Keybase's exploding messages is that when they expire, the text is replaced by the md5sum of the message. So a faked screenshot can (potentially; I haven't verified this) be proven to be faked by appealing to the md5sum in its place, crucially, without needing to reveal the contents of the original message.
Exploded messages are just replaced with an image of what people are calling 'ashes'.
Further conversation on KB about this points out that hashing the message would compromise the secrecy. I still think it would be a neat feature.