In the last beta https://dl.dropboxusercontent.com/u/13064071/plantuml.jar
You can have:
You can use images <img:data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg==>
Note that base64 has to be on a single line. We will try to add the hability to have data on several lines.
Thanks for the suggestion!