Forum Moderators: phranque
It will definitely screw up anyone tracking opens. As far as the images, if your mail looks interesting enough, you can only hope that the user will download the images.
My plan is to keep the most essential image information as small as possible but include it with the mail (title bar images, for example); anything not absolutely essential to viewing the mail properly (or establishing credibility) will continue to be hosted.
You can still get a pretty good feel for the content of the mail even without images being downloaded. But it will be interesting to see how this impacts effectiveness. Subject and key text will become even more important.
<<added>>I think you will want to stay away from any images that take up a lot space (design-wise) - the less space is missing because of images not being downloaded, the more true your layout will be to your original intent. I hope that makes sense :) <</added>>