Google has confirmed that if you want your images to rank in Google’s image search then it is important to use alt tags on your images. Adding alt tags is also important for accessibility. Screen readers use them to describe images to users who are blind.
Alt text is extremely helpful for Google Images — if you want your images to rank there. Even if you use lazy-loading, you know which image will be loaded, so get that information in there as early as possible & test what it renders as.John Mueller, Webmaster Trends Analyst as Google
If you’re using WordPress and have hundreds of images without alt tags there is a useful plugin called SEO Auto Image Tags which generates alt and title tags based on the file name.