I had a look for you, and I can't find a way to do it.
You can add a photo to your contacts on Windows Phone but even that doesn't appear to synchronise back to the people app. I can't see an option on outlook.com either.
They need to update these core apps because at the moment they're really basic.

I think the argument is that it's really down to the user to set his or her own picture. It's a bit idealistic but I can understand the logic.
Which isn't good for any contact you have that's over 50 and will never set it.