If you create a lot of youtube videos, you might spend a lot of time looking for thumbnail images that induce a click.
Here's the “press play” image we use for our videos at the Really Simple System:
Feel free to right click “save as” in order to download this image, or just bookmark this page so you can come back to it when needed.
In case you don't know how to add a thumbnail, you do it on the “edit video” page as shown in this screenshot:
Also, here's a video explaining a little bit more about how to add a Youtube thumbnail image: