Notice: We are aware that many of the Chewiki’s images are still broken. We promise: we will try our best to fix it, but we don't guarantee that the fix will be trivial.
Subliminal Images
From Chewiki Archive - YouChew:
1% Funny, 99% Hot Gas

In Brief
Hidden images is when someone inserts or flashes images/an image onto the video, either for no reason or for a gag. One popular pooper that uses this a lot is LinkOnDrugs.