Image alt text is a must if you have images on your website. It tells search engines what the images are about.
If, for some reasons, when uploading images to WordPress, you forgot to enter alt text, then Slim SEO will help you to fix this automatically.
Slim SEO checks if an image is missing alt text and automatically use the attachment title (in most case, it's the file name) as the alt text. It works for all post thumbnails and the images that you insert in the post content.