The snippets I found online to replace svg with png for legacy browsers using Modernizr were all a little flaky, so here's what I wrote.

replace-svg-with-png.js

// Use Modernizr to check 
if (!Modernizr.svg) {
    // Using jQuery
    $('img').each(function () { // Could also be more specific if you know where the svg's will be e.g. $('#work-content img')
        var $img = this, // Cache current image DOM element in a variable
            src = $img.src.split('.'), // Split the source
            extension = src[src.length - 1]; // Make sure it's the last part of the split string

        if (extension === 'svg') {
            src[src.length - 1] = 'png'; // If it's svg, do all the things
            $img.src = src.join('.'); // Join it back up and change the img source
        }
    });
}