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/