Gmail proxies images, if you send everybody the same image you will get very little information about who is grabbing the image and when (i.e. you'll be able to tell when google (re)populates the cache which gives some small indication that your email is being opened).
This indeed prevents me from tracking. I should have been more clear that my "real" goal is that privacy-sensitive readers will be able to see images. I think these people won't know that the image isn't unique, and so won't load the images.
