Should alt text be added to an image of a person for WCAG compliance?

Question

I am trying to comply with the WCAG accessibility rules for alt text attributes. The decision tree at the following link explains when alt text attributes are required and when they are not: Alt text decision tree. I am wondering whether an image of a person in a biography (e.g. in a company's "Our Team" page) must have an alt text attribute for WCAG compliance. I'm uncertain because I wouldn't describe an image of a person as "purely decorative", but I'm not sure if I would say it "contributes meaning" to a page either.

Context

I set up a web page for work using a WordPress plugin (Team Members) which doesn't allow alt text to be added to images of people (because the images are CSS background-images), and am being asked to add alt text to the images before the site can be put online.