Header/Leaderboard Ad

How can I insert an ad in the header area using a plugin(currently using WP insert and simple ads manager plugins)? And how will I center the ad in my header in mobile version when I turned off the responsive layout in the theme? When viewed using desktop, the ad can be seen at the center, but not in mobile or tablet. Here’s the code I’m using (I switched the plugin off since it doesn’t look good in mobile version).

<div align=”center”>><img src=”http://merrytomarry.ph/wp-content/uploads/2015/08/AMagicalEvent_Platinum_728x90-2.jpg&#8221; alt=”AMagicalEvent_Platinum_728x90 (2)” width=”728″ height=”90″ class=”aligncenter size-full wp-image-6825″ /></div>