To achieve desired effect, you have to send e-mail in HTML format with a picture "embedded" into it (rather than attached as a separate file).
The exact way of doing this depends on your e-mail client program (like Outlook Express) and e-mail service provider. Most major free web-based e-mails (Yahoo, Hotmail, GMail, AOL) do not support this if you compose message via web interface. However you can do it with GMail or Yahoo Mail Plus over SMTP.
In Outlook Express, just use 'Insert Picture' button when composing a message and paste your picture into the message body. Make sure you have selected 'Rich Text (HTML)' format as message format.
In other e-mail programs it should be very similar - the idea is to use HTML/Rich Text format and paste the picture into its body.
Use picture only in JPEG or PNG format and make sure it's scaled down to reasonable size (1024x768 or so) that will fit on most people's monitors.
It's a good idea to include some text at the end to indicate what is this about, as many e-mail providers will initially block any images included in the e-mail.
If you have technical ability to do that, it's also a good to include a link to the web page with the same content so that people who can't read "rich" e-mails will have alternative way of looking at your message.
There's nothing inherently bad in embedding a picture into e-mail and there are many legitimate reasons for doing so.
True - many spammers employ similar technique, and some corporate firewalls may block such e-mails as spam, but that would not necessarily prevent your e-mail from reaching most of intended recipients.