Place it at the exact cut where the image disappears, after a shocked sound that sets up the moment and before a still frame that lets the disappearance sink in. Follow it with a soft thud or a single laugh only when the joke needs a landing, not as a default ending.
About this sound
This is a very brief, roughly one-and-a-half-second scream built for the emoji-turns-to-dust gag, sharp and over-the-top as a pictured face breaks apart and disappears. It feels like sudden panic mixed with cartoon shock, funny rather than frightening, because the outburst ends before it can turn into a real scare.