the image isn’t on your site, the them is pulling it in from a location called mcssl.com
go into your theme’s header.php file (or better yet create a child theme so that when you have to upgrade the original theme you won’t have to do this again!) and find
<!-- Top Ad Starts -->
<div id="topbanner">
<a href="https://www.1shoppingcart.com/app/?pr=1&id=173867" target="_blank" data-bitly-type="bitly_hover_card"><img src="https://www.mcssl.com/Netcart/banners/banner15.gif" border="0"></a>
</div>
<!-- Top Ad Ends -->
and delete this bit
<a href="https://www.1shoppingcart.com/app/?pr=1&id=173867" target="_blank" data-bitly-type="bitly_hover_card"><img src="https://www.mcssl.com/Netcart/banners/banner15.gif" border="0"></a>
once you’ve done that the image should be gone and you can now add in your own image to the same location.
<!-- Top Ad Starts -->
<div id="topbanner">
<a href="https://siteyouwanttolinkto" target="_blank"><img src="https://www.youriste.com/location of image" /></a>
</div>
<!-- Top Ad Ends -->
you’ll need to tweak the CSS under the #topbanner rule to ensure that it displays at the right size and position as that’s still set for the original image.