You can import available sprites from the PlantUML STDLIB (http://plantuml.com/stdlib).
You can import available Archmate sprites (http://plantuml.com/archimate), sprite could be read from embedded png file:
sprite $aService jar:archimate/application-service
You can also import from your filesystem:
@startuml
sprite foo1 sprite1.png
Alice->Bob : hello <$foo1> there
@enduml
with last beta https://dl.dropboxusercontent.com/u/13064071/plantuml.jar
You can have:
@startuml
sprite foo1 some_file.jar~sprite1.png
Alice->Bob : hello <$foo1> there
@enduml
Where ~ delimits the file name (could be zip or jar) and the entry name in the zip/jar file.
This has not been highly tested...
You can also import SVGs:
sprite foo1 some_file.zip~sprite1.svg
sprite foo2 sprite2.svg