YouTube: Someone is using a picture I created as their profile thumbnail without my permission; What can I do?

[Joseph]

A while ago, I created a photo on photoshop and set it as my profile picture on YouTube. After a couple of weeks, I discovered one of my 'friends' had set it as their profile picture. After an argument, I was blocked on facebook but eventually got that picture taken down as it belonged to me. Now I discover that he is using that same photo that I had created as his YouTube Profile's Thumbnail picture without my permission. I want that photo to be taken down, as it belongs to me. What can I do to achieve this?
I flagged the profile thumb. as that was seemingly the only option under report that was appropriate, but all I could do is click the button and flag it, without adding any details. I'm pretty sure this won't work, since YouTube don't know the story, so my question is what ELSE can I do that will be effective?