The recommended banner size changed in the update. Now it's 1000x220px. Unfortunately the bit telling you what the size should be wasn't updated, so it still says the wrong size:/
Alternately, you can use the following in your CSS that will auto-size things nice and happy regardless of the size or proportions of your banner, which is important because your banner will be used at a smaller size in the private sections of your site. Note of course that this will stretch your image if it's not big enough, so you probably will want to come along with a higher resolution version eventually, but this will get you back on the road and let you keep your proportions as you like.
Comments
/* CUSTOM BANNER SIZE */
.campaign-public-layout #banner-container .campaign-banner-image {fixed; height: 298px; width: 744px; margin-right: auto; margin-left: 240px;}
If you are not, the only recommendation I have is to format your banner or create a new one as close to 1000x220 as you can.
Unconquered Kingdoms, July 2016 CotM
.campaign-banner-image{ background-size: 100%; background-repeat: no-repeat; }
Example: "main page":https://clarity.obsidianportal.com/