Alt text and SEO

Hi all.

I had a client query about image alt text on decorative images relating to SEO. They're obviously coming at it from an SEO perspective only. They're happy to null alt them for WCAG/accessibility purposes, but still question the SEO benefits. I know that including alt text may also contribute to image SEO, yet we hide them using the alt attribtue. So, my question is, will a null alt nullify any SEO ranking for said images? Should we not worry about it? Is there another way of hiding the image that I am missing that will hide the image, but still have the alt attribute? Or am I thinking too much into this? Apologies I have very little knowledge on SEO and how it actually works.

Much appreciated as always,

Tom