Problem/Motivation
The drupal_html_to_text function removes images throwing away the alternate text. This is arguably bad from an accessibility perspective. Images could contain semantically meaningful information which is expressed in the alt attributes. In addition, I did a quick comparison and mailchimp appears to add alt attributes from images to their plain text versions by default. It is a little different with them as their plain text is editable. However, if the industry leader is doing it by default, it's probably a good idea.
I suggest replacing images with alt text when alt text exists. Below is one proposed solution (patch coming d7/d8).
include/mail.inc (d7)
401: $supported_tags = array('a', 'em', 'i', 'strong', 'b', 'br', 'p', 'blockquote', 'ul', 'ol', 'li', 'dl', 'dt', 'dd', 'h1', 'h2', 'h3', 'h4', 'h5', 'h6', 'hr', 'img');
...
414: // Replace images with alternate text
415: $string = preg_replace('@<img[^>]+?alt="([^"]*)"[^>]*?>@i', '$1', $string);
Note, there is a related 250 commenter, but it seems to be inactive (2011) and didn't appear to mention image/alt handling.
See
#299138: Improve \Drupal\Core\Utility\Mail::htmlToText() →
Proposed resolution
Remaining tasks
User interface changes
API changes
Data model changes
Release notes snippet