Use a healthy balance of text and images in your emails and newsletters. And stay away from remote images. These are images that reply to the sender when an email has been opened and most email apps can find these and warn the user that they are present and it makes you look underhanded because you are not asking the user for permission to send this data.
In addition, these email apps by default do not automatically load images unless they have been pre-approved or loaded on a case-by-case basis and if the user chooses not to load your images, you will not know they opened the mail but chose not to load the images so the data retrieved is somewhat inaccurate as to stats etc.
Instead you could embed a link that lets you know they read the message, but does not redirect the user to anywhere, if it does then they will likely get ticked off too.
If you embed your text and graphics into a single image, you risk your message going straight to the junk folder. In addition, since most email clients suppress images by default and if your email is one big image it will like open as a blank email with no call to action. Even worse, the first thing your recipients might see is your unsubscribe message. Ouch.
If you intend on sending a text-only message, design it as a text only message from the start. If you design your message as text in an HTML document, this could also cause issues since filters will see the email unevenly weighted between html content and text content causing potential filtering.
Even if you are not building your email for business to business, these are all good ideas for consumer driven emails as well.