Alt text for header image

Recently, I deleted the image I use for header image in the email template, not remembering its purpose. I was surprised to see emails that said “Offshore Dive Center” at the top instead of the image (this text has nothing to do with my site).

I went back to older emails and saw this alt text was in the raw source even before I deleted the image. I searched for the text in my entire site. It’s in all the emails, but not in any theme or plugin or WordPress file that I could find, including this plugin.

1. How does this text get selected for alt text?
2. How should I set/change the alt text?