Ever since deciding to permanently disable images on all the emails I receive (even from people I trust), I occasionally run into the inconvenience of viewing an email with a bunch of blank squares where I suppose a huge image is supposed to be. Just an image, and almost no text to explain what the email is about. This is a shame because I do sometimes subscribe to newsletters on purpose because I want to stay up to do date with a service that I am using.
Now, I am definitely not going to enable images just for these emails. I disabled images as a matter of principal against all of the tracking that happens when an email is opened. But you can write a multipart email.
Basically, you can send a rich HTML email, with a plain text alternative for those of us who prefer plain text. Rails already supports multipart emails and I imagine many other web frameworks as well. Services such as Mailchimp also support multipart emails so there really isn't an excuse not to include a plain text version in your next newsletter.