How detailed should we get describing a picture (placed for design) or a headshot of an individual for Image Alt text?
There are two keywords to remember – Content and Function. You want to present the core content of the image and, if it is clickable, what the function is.
Alt text is required for proper ADA compliance.
Decorative images
Require alt="" in the actual code, but does not require any alt text... since it does not provide any useful information or function.
Complimentary images
Compliment or support the textual content on the page. These images should be described to someone who cannot see.
Functional images
Perform an actual function when clicked. In this case, you would not describe what the picture looks like, but rather describe the function when clicked.
One question that can be helpful when considering proper alternative text is “If I could not use this image, what text would I put in its place?”
For a headshot or "portrait" image, typically the person’s name will suffice for alternative text.