Displaying an add differently on a mobile device

Hello, I am using the latest version wordpress and the twenty twelve theme. I have a child theme set up, my website is https://www.cryptorushnews.com. The following add shows almost perfectly in google chrome but on my android Samsung Charge it overlaps the header logo image and makes for a very messy page.

CSS

/*Place ad banner to the right and add top margin*/
.header-ads {
    position: relative;
    top: 30px;
    float: right;
}

headher.php

<div class="header-ads">
<a href="https://cex.io/r/0/cwmills/0/" title="Bitcoin Mining and Trading" target="_blank"><img src="https://cex.io/img/b/728x90.jpg" width="628" height="90" border="0" alt="CEX.IO"></a>
</div><!--/ .header-ads -->

How do I make this look better on an android or iphone/ipad device?