I had thought sending attachments would be preferred to linking to images on the website because viewing a message wouldn't require an internet connection, and wouldn't provide a log entry on the web server (some people are concerned about tracking), but the additional bandwidth required to send tens of thousands of messages isn't worth it. Plus, people are accustomed to some images being linked into an email message and modern email readers let the user decide whether to download an image. Some systems, like Gmail and other large cloud email services, proxy images anyway.
The tracking concerns then don't have anything to do with the IP address from which the reader access the image, anyway. Besides, the way marketers accomplish tracking is by adding some unique identifier to an image so that they know the recipient opened the email message -- I'm not adding any such crap to our newsletter messages.
Usually, the only image included in the newsletter is this K logo: