Image alt text purpose
When looking at making sure your image alt text works for your SEO purposes, don’t forget that one of the main purposes of alt text is to provide visually impaired people a description of the content they’re looking at. It can be easy to get carried away filling it up with SEO-oriented keywords, leaving it useless for screen readers and affecting the readability of your website for those who rely on screen readers. So, that said… When looking at your image alt text for SEO purposes, is longer better? Not necessarily!
Current thinking regarding alt text
For visually impaired readers, however, there may be a benefit from having longer alt text component to the image – something to keep in mind when looking at your content. From the Penn State University link it seems that some screen readers break the text up into 125-character chunks, though, so longer than that limit may not work well anyway.
Images that aren’t part of content
If you don’t want to put alt text in for every image or have on-page images that aren’t part of the content, at least put in an empty entry – e.g.
img src=”blah.jpg” alt=””
That way screen readers will skip it and not say “image”.
Modules/plugins relating to alt text
There’s a Drupal plugin which scans your website for missing alt text; it’s a dev release but it may be worth looking at if you’re trying to revamp your site and ensure that the alt text is present and appropriate.