Here's a high-level summary:
Based on the results of a survey conducted by EmailLabs we know that (besides the sender name & subject line) the top left 2-4 inches (5-10 cm) are the most valuable real estate in an email. Why? Because that is the only area visible in both horizontal and vertical preview panes. Think of this space as the teaser area and use it to grab the readers' attention and to compel them either to scroll or click to stories or fully open the email. Therefore you should redesign the top of your emails so that key content can be viewed in the preview pane even if images are disabled. If you have to use images, at least make sure that you ALWAYS include a descriptive ALT tag.
The full article includes the survey results from the EmailLabs survey and a lot more useful tips, so it's definitely worth reading.