>As a general rule it's a good idea to avoid looking at any messages you might suspect are spam.
>
>Most spams are HTML; when you view them your browser is invoked behind the scenes to render the message. To do so it goes out to the web site of the spammer to download images etc. The spammer's site almost certainly records the IP addresses of any computer that downloads its images. If the spammer has any sort of cross-reference between IP addresses and e-mail addresses, now she knows you're a "live" one and will spam you all the more.

Which is why Mozilla Thunderbird by default doesn't show the images in suspect messages.