It looks like google gmail now supports Japanese decome (デコメ) or decorated mails. These are small charset extensions added by Japanese mobile companies to allow cell phone users to exchange emotional icons in their email.
Example with a peace sign
Decome is specific to japan so most computers will not support this natively. To implement this It seems google have a public image repository which is referenced to display the pictures. This should prove useful for 3rd party mash-up applications as well.
Code example
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今帰るので、Checkしてみます
<img src="https://mail.google.com/mail/e/docomo_ne_jp/B94"
goomoji="docomo_ne_jp.B94"
style="margin: 0pt 0.2ex; vertical-align: middle;">
<br>Pretty nice stuff.