Why do you have to specify the padding-bottom? Is it the image ratio (as 16:9 = 56.25%)? In that case, it would practically render the plugin useless for sites with many images with unknown size.
At the same time, one could use something like ImageMagick to find out the dimensions of the picture and set this ratio programatically (http://stackoverflow.com/questions/16266933/rmagick-how-do-i...)