Since SIkuli is a screen capture and replay based testing framework, it needs access to the Windows desktop. Running as a Windows service does not give access to the Windows desktop by default.
I suggest that you should configure a dedicated Windows machine that is logged into the Desktop automatically when Windows starts and automatically starts the JNLP based agent to connect to the Jenkins server. Assign a label to that agent (maybe 'sikuli') and run your sikuli tests on that agent.
There are other techniques that attempt to give Windows desktop access to a Windows service. I'm not persuaded that they are worth the effort. Windows services generally are not expected to interact with the Windows desktop.
Mark Waite