Ah, I see what you mean -- I searched for Princess Luna, and it does look like you changed her profile picture, since it's is now the same one you have. It should be okay; you can message her and explain the mistake. It should be easy to fix.
Just let her know that you accidentally signed on as her and changed her profile pic, and anything else you remember changing. Funny that you have the same password. What are the odds!
To find her and message her, look at the top of the page and find the link that says "My Messages." Hover over it and click on the "Send a Message" option, and then it will take you to your message form. Put "Princess Luna" in the "To" space. Fill in the form, letting her know what happened, and you'll be good!
Mods or others, feel free to add if I missed something or if there's a quicker way to fix this.