In the last beta https://dl.dropboxusercontent.com/u/13064071/plantuml.jar
You can have:
@startuml
start
:foo;
note left
You can use images <img:data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg==>
end note
stop
@enduml
With size definition:
@startuml
start
:foo;
note left
You can use images <img:data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg=={scale=3}>
end note
stop
@enduml
Note that base64 has to be on a single line. We will try to add the hability to have data on several lines.
You can also do:
@startuml
start
:foo;
note left
You can use images
<img:http://plantuml.com/logoc.png{scale=2}>
end note
stop
@enduml
And also using creole engine (see http://plantuml.com/creole.html ), you can have:
start
:foo;
note left
You can use images
<img:http://plantuml.com/logoc.png>
end note
stop

Is this what you are looking for ?