Nope, I send a ton of emails with images in the body which regularly have a spamassassin score of 0.3 out of 4. Use your regular <img src="x.jpg"/>
. Just make sure you have a good amount of real text.
If you're a 'trusted sender', meaning you send a high volume of emails that people don't mark as spam, you can get away with all images. Examples of companies that do this off the top of my head are Seamless Web and Lego.
I could see base64 code either being stripped or setting off the spam alarm though, based on how many hack scripts there are that use base64 to obfuscate code and then run it.