The best guidelines I have found for crafting HTML emails supported by all major clients:

Campaign Monitor Email Design Guidelines (scroll down for their discussion on images)

http://www.campaignmonitor.com/resources/will-it-work/guidel...

Image Blocking:

http://www.campaignmonitor.com/resources/will-it-work/image-...

CSS Support:

http://www.campaignmonitor.com/css/

The one I use does, yes, and probably most of them do. And I really like this feature. But I'm certain that if I wanted to support a candidate and subscribed to his newsletter, I would allow all images from sender because I would want to see the pictures, or just because the newsletter layout looks better with images.

And people managing these kind of newsletters probably know that. They design nice layouts for their newsletters, with images and fancy stuff to be "visually friendly". And they know people will want to let pictures in, and they know they will then have the possibility to track views. They even include the infamous if this email isn't properly displayed, see it here [http: //link/mail/online/?here_is_who_clicked_this_link=your_name] (usually more subtle than that.)

I was testing out mailchimp the other day, and I could not for the life of me get the thing to recognize that I had read my test e-mails to my accounts, even though I had.