Various thumbnail sizes

It isn't possible to automatically generate two different sizes (738x388 and 131x70 for example) for video thumbnails.

By default, 2 different thumbnail sizes are generated for each video. However their sizes aren't directly cutomizable. The first thumbnail has a fixed size of 200x120 pixels and the second depends upon the encoding resolution you want. In other words:

1. Small sized thumbnail: 200x120
2. Big sized thumbnail: maximum output resolution requested

The size of these thumbnails doesn't depend upon the source but upon the exporting formats that have been set. The preload image takes the size of the largestt format to be encoded. This allows to have a preload picture corresponding to the video that you wish to integrate (same resolution, format, ...).

For the other resolutions, you can retrieve the maximum picture via Curl/wget and resize it on your website.