Help with responsive banner code in House Ads on mobile

Hello friends,

we use the code below to show in house responsive image ads

<a href="MY WEBSITE LINK" target="_blank">
    <img src="IMAGE LINK" style='width:69%;' border="0" class="banner" alt="Null">

on the desktop is fine but on the mobile view, the banner is showing very small. see the below

!! the filed only accepts HTML.

What’s your point?

How does that prevent you from using @media?

am sorry i don`t know how to use it.
can you advise?