The Gmail option 'Ask before displaying external content' does not work as expected, i.e. external content such as web bugs gets displayed anyway. The linked screenshot shows an example:
http://oi48.tinypic.com/2uqoroj.jpg
I used Reeder to mail myself a New York Times article. I usually use Mailplane to access Gmail and Mailplane enjoys free web access. After an update of my personal firewall Little Snitch, however, I had to reassign access rights and Little Snitch showed a request to nytimes.com. That is what I call 'displaying external content WITHOUT asking' …
The issue is reproducible in Mailplane as well as in Gmail in common browsers such as Chrome, Firefox and Safari.
Apple Mail, by the way, does not try to load the same external content:
http://oi47.tinypic.com/6ieams.jpg