M&R Strategic Services did a quick little study on the effectiveness of images in advocacy emails. Basic idea is now that so many people block images in their mail clients, or their mail clients do it for them, images may hurt, rather than help the organization.
You can read the 4 page study here.
From what I gather, the numbers don’t prove the case “images hurt you now.” And the study doesn’t answer the question in the title “Do Images Help or Hurt?”. The number say “The effect of images in email are really statistically insignificant.” The differences in open rate or response rate were all withing a few hundredths of a percentage point, which given the unreliability of email metrics like open rate tracking and click through tracking, I would say are within the margins of error.
The authors of the report admit this:
Although we cannot make any definitive claims based on the results of just three tests, these results do indicate that the inclusion of additional images does not increase response rates to advocacy messages or fundraising appeals, nor does it decrease them. While it may not hurt to include properly formatted images in your email message (thought it could hurt if your message gets quarantined or rejected by a potential donor or activist because it looks blank in the preview pane), it also does not appear to help at all.
What might have helped here is talking to some of the recipients, and also doing a bit of analysis on what caused message to be quarantined or flagged as spam.
The report does offer some good common sense advice if you are going to use images in your email messages.