A just-released study from “The Email Experience Council” (EEC) says emails optimized for image blocking positively impact revenue by raising the roi per dollar spent on it to $52.69 from $48.29.

The study of email design practices of “104 top online retailers”, and 472 marketers revealed that “more than 50% of the images in promotional emails are routinely blocked by email and web mail programs,” and “23% of retailers send emails that are completely unintelligible when images are disabled.”
Source: eMarketer