PictureFill - showing no image on mobile devices

Question

I am using picturefill (https://github.com/scottjehl/picturefill) for responsive images.

I have a page where I would like to show an image when above 750px, but not below (i.e. not load the image at all on mobile devices).

My code:

<picture>
    <source srcset="myimage.png" media="(min-width: 750px)">
    <img alt="Indicator Selection" class="step-img">
</picture>

This code kind of works - in that the src is not loaded on smaller screens (just the alt text shows). Ideally, however, I'd like for the browser to not even look for an image on mobile. How can I achieve this?