Here is some simple jQuery to use alongside Modernizr to replace IMG tag src file extensions from SVG to PNG. You’ll need to drop in a png copy of the image with the same name in the same directory as the SVG file.

if(!Modernizr.svg) {
    $('img[src*="svg"]').attr('src', function() {
        return $(this).attr('src').replace('.svg', '.png');
    });
}