I’ve just recently advised a client to make ensure there is some plain text within their email campaigns, rather than just embedding one large image as the entire promotion. My main reason for this advice is that email programmes often turn images off in emails. So if you’ve got just one image as your entire campaign there is a chance some users will get a blank email (unless they turn their images on).
As if that’s not harmful enough to your marketing efforts, there can be even more dire consequences, with people mistaking the email for spam (that’s assuming your campaign even makes it through a spam filter). If you’re reported for spam you could then end up on a black list.
For a more detailed explanation and more advice on how to avoid this potential problem, check out the recent MailChimp blog post on the matter.