johndyer/mediaelement — pull request #838 by eyefood (2013-04-14 15:57:15+00:00)
Title: Proportional poster images (IE9+ Chrome, Safari, Firefox)

Added the poster image as the CSS background image to the .mejs-poster div, and hid the original from all browsers except IE8 (where it's still 100% wide and high) and below with the following CSS

:root .mejs-poster img { display: none ; }

Added the following CSS to constrain the background image to the div:

.mejs-poster {
background-position: 50% 50% ;
background-repeat: no-repeat ;
background-size: contain ;
}

That should keep the poster image constrained and centred vertically and horizontally, so that it covers up the video element nicely.