<table class="emailWidthAuto" width="600" border="0" cellpadding="0" cellspacing="0">
    <tr>
        <td class="banner" align="center"><img src="http://unsplash.it/g/1200/400?text=BANNER" border="0" alt="Banner"  style="display:block; width: 600px" width="600" /></td>
    </tr>
</table>

@media screen and (max-width: 600px) {
    .emailWidthAuto {
        width: 100% !important;
        height: auto !important;
        min-width: 100% !important;
        max-width: 100% !important;
    }
    .banner img {
        height: auto !important;
        width: 100% !important;
    }
}