I had one of those moments today, when you have a clear image of settings or functions that do not exist.  We are working with image files in our Drupal website, and wanted to require an alt field for every image that is added.  Just check the box, right?

There's no alt field required box in Drupal 7 when using an image field.  

So how do you force content authors to use allows Alt tag for images?  Enter the EIM module - Extend Image Module.   It's a small simple module that populates image field settings with a required field.  If accessibility compliance it critical, this module is too.  

Give it a go.