You can't add images or upload custom styles to WithWine's email templates. WithWine keeps email templates deliberately generic and simple, for these reasons.
Why email templates are kept simple
Clear calls to action. Each email has one job, such as paying an invoice or making a club selection, and a clear button to do it.
They work in the WithWine app. Customers who use the WithWine app also get the text of each email as a message in a chat bubble. Images and styles wouldn't carry over to a chat message.
They display reliably. Simple emails look right in every email app and on every device.
Want a designed email with images?
To send newsletters and marketing emails, use the email builder or HTML composer on the Customers or Club Members screen instead.
What you can change
You can change the wording of the pink highlighted sections and the subject line. See How to change the wording of an email template and How to change the subject line of an email template. Your logo appears at the top of each email automatically.