Hi,
HikaShop doesn't attach any attachment to the emails sent to the customer by default.
So I suppose that you're actually talking about the look of the email HTML content itself.
For that, what you can do is edit an email in the menu System>Emails. There, you'll find an option "template" where you can select a template for the email. Next to it, you have an edit button which will open a popup where you can modify the template.
In that template, you first have a bunch of CSS for the colors of the emails, and then the HTML code around each email.
So you can add images in there. Just add the img HTML tag of your image.
Then, save the template, save the selection of the template, and you can use the "preview" button at the top to check how the email will look like with your modifications.