Hmm replace your current HTML:
<div align="center">
<a href="http://bit.ly/2soUfDo"><img src="http://www.artofboardgaming.com/wp-content/uploads/2018/06/NEWjune.jpg" alt="" width="900" height="300" style="padding:20px"></a>
</div>
To this:
<div class="banner">
<a href="http://bit.ly/2soUfDo"><img src="http://www.artofboardgaming.com/wp-content/uploads/2018/06/NEWjune.jpg" alt="" width="900" height="300"></a>
</div>
Then add this CSS:
.banner {
text-align: center;
padding: 20px 0;
}
Adding CSS: https://docs.generatepress.com/article/adding-css/
Let me know if this helps π