How to know if someone is trying to access my PC unauthorizedly?

My friends are using my laptop whenever they visit me at home. I was shocked when I saw some of my original pictures stored on PC are showing on a search engine.
I asked my friends if they touched my files, but they said they didn't. I trust them very much so I started thinking maybe someone is trying to break into my computer anonymously, and that's why someone was able to get those pictures.
Is there a way to check if someone is trying to access my computer unauthorizedly?
Please let me know your inputs about this?
Any suggestions are very much appreciated.
Thank you very much in advance.
