Back in May the Email Experience Council wrote about Image suppression in the web clients for and and how you should expect to see a drop in open rates because of this.

Almost every web mail provider suppresses images for users by default now, and some email clients have been for years, and marketers need to understand how this will effect their communications to their clients. Your team should be coding each and every email with the idea that your images will not display. Use some of the tips provided in “Now you see them, now you don’t” to help with image planning in email.

MarketingSherpa recently promoted a case study focusing on this exact issue. This study provided four key guide lines for using images in email resulted in a 41% increase in clickthroughs.

Email Marketing Reports also has a great resource for image related suppression.

Look to the tools provided by email Reputation Service Providers (eRSPs); Habeas, Return Path and Pivotal Veracity for content rendering test tools.