Lead Developer
Alt tags don’t show up when you hover over the images – that’s the title tag.
It looks like your images have alt tags, which means Yoast is just throwing you a false negative.
I don’t think you have anything to worry about 🙂
Documentation: http://docs.generatepress.com/
Adding CSS: http://docs.generatepress.com/article/adding-css/
Ongoing Development: https://generatepress.com/ongoing-development