For each video there is a fix set of 10 thumbnails. One custom thumbnail and 9 automatically extracted thumbnails.

These 10 thumbnails have fixed URLs for each movie. Whenever you upload a new custom thumbnail, you replace the image referenced by the URL for the custom thumbnail.

When a thumbnail is selected for a video, one of these URLs (or the account default thumbnail) is picked as the new thumbnail. So new URLs are not created, only the selected one is changed.