Guess what? All email is ASCII text-based!
When you "attach" an image or other non-text content to email, it is hex-encoded so it can go through the SMTP system, which is ultimately defined in RFC 822 and descendants as consisting only of 7-bit ASCII characters.
This might only be an idle bit of trivia, except that, believe it or not, there are still people on slow connections. If you send them a 100k image, it comes over their modem as 200k worth of data, because the one-byte binary value "256", for example, must be encoded as two bytes: "FF".
I have aDSL here, but it is horribly slow, sometimes rivalling dial-up speeds. (Dial-up slows? What's the opposite of "speeds?") I do not appreciate big, non-text emails, and I generally unsub from sites that thoughtlessly insist on sending me "updates" that can take minutes to arrive.
Worst of all is the clown who was sending me HTML "updates" that included a megabyte image that was then scaled down to 180x240 via parameters in the HTML "img" tag. Needless to say, they no longer "update" me!
And if you're thinking Flash, fuggedaboutit -- I have Flash permanently disabled for email, and even have a "click to load Flash content" extension in my web browser of choice. Someone sends me Flash content once, they get unsubbed. Send it twice, they get reported to spam blackhole lists and blocked in my firewall.
I agree with others here: email should be shortish and primarily ASCII-text based. For other content, send them a link to your site -- perhaps with a smallish image, if you must.