I realize that this is a 2012 question. The "most email clients don't support this", and the "don't do it, you'll be labeled a spammer" response is incorrect. My bank, Microsoft, many newsletters, and most other businesses link to images on their sites. Right click images in most business emails, click copy image address, paste it into the address bar, and you'll see that the image is hosted on their server. This is why many old emails are missing their images. They've been deleted off the server.
Being labeled a spammer has a LOT more to do with return path certification, and properly setting up your email servers.
https://mail.live.com/mail/services.aspx
Per my host, not all tests have to pass: https://intodns.com/