Kwidget.thumbEmbed custom size thumbnail

I’d like to have some clarifications on how the kwidget.thumbEmbed() chooses the thumbnail to display - before the player is loaded.
I have a player sized 450x253, and the thumbnail is always narrower (it keeps the height, but fits the width for the ratio of 4:3). I’d like to have a thumbnail that always fits the sizing specified for the player - or that it works in a similar way as the kaltura thumbnail api.
I’d like to know also if it’s possible to customize the “Play” button, using my own .png file.