Alt text?

I am required to have our website comply with WCAG 2.0 Level AA and the checker keeps telling me that I need to have an “alt” tag assigned to the image that I am using as the full screen background image. The image itself in WordPress does have alt text, and I have tried setting it to decorative, but this checker wants alt text, and the full screen background image plugin seems to not pick up the alt text from the image. I have read that a background is decorative and does not need alt text, but tell the checker that! Is there a way to placate this requirement and just get a word or two of text included with the link to this image that I am using for the background? Thank you.

https://www.ads-software.com/plugins/simple-full-screen-background-image/