Ran across a neat work around for customizing ArcGIS online and thought I would share!
When editing your organizations home page (My Organization -> Edit Settings -> Home Page), I found that the banner section allows for HTML! This is great as it allows for a bit more customization of the homepage display. I put some basic HTML in there at first, allowing for me to have a taller banner. After that, I wanted to push the envelope a bit because my page background image was cool, but the cooler clashed with the footer links, so you couldn't read them! So, I put a <style> tag in the banner WYSIWYG, and it worked! You can see what I did in the image below. Also, the HTML is:
<div style="background: url(https://www.yakimawa.gov/images/esri-online/banner2.jpg); width: 960px; height: 470px; border-radius: 0px 0px 5px 5px;"></div>