Mostly promotional email contains lots of images or at least one. So if you are using Gmail's tabbed inbox and you haven't disabled promotions tab then you'll be able to get in the field trial for Gmail.
Go and apply for field trial and if you are selected you'll be able to see thumbnails for emails in your promotions tab. You'll be able to switch to list view that is current and grid view that'll be added to your Gmail account (grid view will show thumbnails).
It will help you to see which promotional email is getting you something that you wanted, by showing thumbnails of course.
Official Gmail Blog: A new, more visual way to view your Promotions tab
