Your browser does not seem to support JavaScript. As a result, your viewing experience will be diminished, and you have been placed in read-only mode.
Please download a browser that supports JavaScript, or enable it if it's disabled (i.e. NoScript).
A number of systems (youtube included) simple give you a predefined selection of thumbnails to pic from.So perhaps a default of position 0% as it is, but with the option of changing a thumbnail to use 10% 25% 50% or 75% as alternative thumbnails.
Please submit a ticket so we can get this logged in our database.
Good find!