Main Header Image ALT Text

Greetings! We were told by an ADA evaluator that our page (ci3t.org) wasn’t in compliance for people with disabilities because the main header image doesn't have alt text. I try to always add alt text to all images, but in the appearance settings where the image is set there doesn’t appear to be a place to add alt text. I added it when the picture was uploaded to Media, but it’s not showing up when displayed on pages (www.ci3t.org). In the CSS I don’t see anything preventing alt text… Is there a way to ‘turn on’ alt text for the header image? Seems like this is something the theme should have by default to support screen readers and people with visual impairments, so maybe an update to the theme for everyone? Thank you! I searched the forum for similar but couldn’t find anything this specific. #header-featured-image, .wp-custom-header { clear: both; display: block; line-height: 0; width: 100%; text-align: center; }