To edit the news letter image
Login to your website, go to the home page, click Edit at top of page.
Scroll down to Newsletter area. Select "Banner" block > click Edit
Select / Upload image
Make sure you add in Alt text for good SEO
You don't have to edit any other details.
Troubleshooting
Image is missing after your save? - Clear your Cache
https://support.tourcms.com/hc/en-us/articles/9344170468369-Gray-Line-Licensees-Clear-Cache-article-