From a marketing perspective, there is little doubt that an image-rich email generates a better response both emotionally and statistically from its targeted recipients than plain text.
Consequently, their use has become more common (and sadly more abused).
Newsletters designed for Internet email clients are subject to the specific limitations and constraints imposed by the email clients, for obvious security reasons.

Read more about how to create HTML newsletter emails that really work in this great document.

Creating HTML email templates for the best possible outcome

Good luck,
Claudiu