How to render alt and title texts of image file entities using a theme function
The logic is pretty easy, you can just preprocess theme_image():

function MYTHEME_preprocess_image(&amp;$vars) {
  $vars['attributes']['alt'] = 'alt';
  $vars['attributes']['title'] = 'title';
}


If you're using any of the standard image display formatters they'll eventually be passed through theme_image() so that should work for all fields.

I'd strongly advise you consider not doing this though. The point of an alt tag is to describe the image to parties who can't interpret it in its original form (visually impaired people, robots, text only browsers, search engines, etc)

By setting the alt for every image to the same string, you're making your site quite inaccessible to those groups of visitors. And since Google and others assign a reasonable weight to alt text, you won't be doing your SEO any favours either.