Gathering Statistics on Blocked Images in Email Marketing Campaigns

A lot of my clients use different email system to handle their marketing campaigns. Whether it's RatePoint, Constant Contact, or some of their own proprietary platforms, they are all running into the same problem -- most modern email systems don't display inline images by default. This gives them the fear that their mailings are giving an even lower return rate to an already low rate. I tend to think that the people who pay attention to email-based advertisements are pretty likely to hit the "always display images from this sender" button, but you can never be sure. Additionally, people tend to differ in these regards from product to product. 

Anyways, I'm going to be inserting some of my own tracking into clients email campaigns to see who's really displaying the images. Simply place <img src="http://myserver.com/img.php" /> line in the source of your mailer, and have the corresponding php file contain this sample code:

The img.jpg could be a 1 pixel square or a real image from your html email, it's up to you. To track, this script simply takes the value of the current # of reads (stored in stats.txt) and adds 1 to it.

I'm curious to see how Gmail or Outlook handles this. Emails that come through Gmail and have inline images display in a box that's the same width and height of the image (until the image loads). So I'm curious if they're doing any pre-loading (which would trigger this script) or if they are doing pre-loading once per request (regardless of how many emails use the same file) and caching the data.