fixnobgsize.jquery.js

/* global Modernizr */
var fixNoBackgroundSize = function () {
  var bg2img
    ;

  bg2img = function ($el) {
    var bg
      , classes
      , $r
      ;

    $el = $el.eq(0); // meant to work with one element at a time
    classes = $el.attr('class').split(' ');
    classes.push('bg2img');
    $r = $('<img>');
    console.log(classes);
    classes.forEach(function (cl) {
      $r.addClass(cl);
    });
    bg = $el.css('background-image').match(/\((.*?)\)/)[1].replace(/('|")/g,'');
    $r.attr('src', bg);
    $r.css({
      width: '100%'
    , background: 'none'
    });
    return $el.replaceWith($r);
  };

  if ( !Modernizr.backgroundsize ) {
    $('*').each(function () {
      if ( $(this).css('background-size') !== 'auto' ) {
        bg2img($(this));
      }
    });
  }
};

Here I was toying around with this idea of looking for images with background-size set and replacing them with an img element. It's not a viable production solution because there are too many ways to break.