Chargebee supports both plain text and HTML based email content and you can easily customize the email templates as needed. You can insert images in the HTML template.
This can be done under Settings > Email Notifications > Enable Email Notifications.
To insert an image or any HTML content in the email template,
Click on Customize Template and go to the HTML tab.
In the toolbar, you can select the insert images tool.
A pop-up window will appear where you need to give the URL for the image. If you do not have an image URL, here's what you can do:
Below the email template, select the Upload image option.
You can choose the required file and click on Upload.
Once the image is uploaded, click on the image and the image URL will be copied.
Paste this URL in the Image URL field and insert the image.
Your image will be inserted in the email template.
You can do a preview and even send a test email to see how it looks. When you're happy with it, you can save your changes using "Update Template".