I was trying to do something similar (embed build status icons into email notifications) but since our build system is not publicly accessible, I ended up posting the status icons to imgur and referencing those images by storing the URL in variables according to state.
Here are some snippets from my email template Groovyscript:
Defining the URLs:
def images = [:]
images["SUCCESS"] = "http://i.imgur.com/uXlqCxW.gif"
images["PASSED"] = "http://i.imgur.com/uXlqCxW.gif"
images["UNSTABLE"] = "http://i.imgur.com/QkQbxR3.gif"
images["SKIPPED"] = "http://i.imgur.com/QkQbxR3.gif"
images["FAILURE"] = "http://i.imgur.com/LUveOg7.gif"
images["FAILED"] = "http://i.imgur.com/LUveOg7.gif"
images["ABORTED"] = "http://i.imgur.com/jSdrWWP.gif"
images["NOT_RUN"] = "http://i.imgur.com/jSdrWWP.gif"
Using the URLs in the email:
<img src="${images[build.result.toString()]}" />