The following example works:
@startuml
start
:foo;
note left
You can use images <img:data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg==>
end note
stop
@enduml
However, trying to use bigger images results in longer data URLs, and PlantUML fails (silently). There's no error, and there's no image. Here's an example:
@startuml
start
:foo;
note left
You can use images <img:data:image/png;base64,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>
end note
stop
@enduml
The Image URL is ok:
It fails silently with the PlantUML.com server AND with the .jar file in Windows. No image is produce and no error either.