For our marketing emails, we want a width that's more appropriate for
newsletter context, vs. the narrow emails we use for transactional
content.
I haven't figured out a cleaner way to do this than duplicating most
of email_base_default.source.html. But it's not a big deal to
duplicate, since we've been changing that base template only about
once a year.
The alt text of the leading images were displayed as preview
content in inbox by email clients like gmail. Since the leading
images were used mostly for decoration this made the preview
content gibberish. It's fine to set the alt attributes to empty
from accessibility point of view since the old alt attributes
did't added any meaningful information.