Replace video frame with bitmap

What method are you using to extract the frame? Also, what type of videos are you working with?

Also, one thing to be aware of, some video codecs do not store the whole frame "bitmap", but rather changes from the previous frame (except for "key frames", which do. These are what allow you to seek videos)

An option you can try - depending on what container you're using is add a secondary video (or audio) stream into the container. Most players won't play them without being "selected". So it'd be relatively unknown, and easy to parse back out