Youtube – How to add a single image frame to the end of the Youtube video

Pause the video. Press . to go one frame forward. , to go one frame back. I use these whenever I use others’ animated videos as references.

I assume you're talking about when embedding the video in your own site:

You can use the Chromeless player to accomplish this. You will have to set up your own play/pause, etc. controls, as well as your own replay button, but that's the trade-off for having full control.

See: http://code.google.com/apis/youtube/chromeless_example_1.html for an example.