It can be hard to create an email that displays identically across all email clients, so you might be tempted to create one large image so that every person sees the same text, fonts, and images. However, there are several reasons why doing this might affect your email delivery rates:
All of these consequences have a negative impact on the conversion rates of your website--if the user doesn't see your email, he or she cannot interact and engage with your content the way you want.
When you send HTML-based emails containing mobile-friendly features, you increase the number of people who can read your emails. This means that it's easier for them to take action--higher click-through rates on your emails directly correlate to higher conversion rates on your website.
While including mobile-friendly features (especially when you take into consideration the needs of all the different email clients available) requires you to spend more time ensuring that your HTML works as desired, the consequences of using images instead are too high to ignore.