IMessage from Canada to Germany?

I have a friend who lives in Germany and I live in Canada. We both have iPhones and mac computers. Would it be free to imessage each other? I iMessaged my friend who lives in California, and I was charged with long distance texts even though they sent as imessage (supposedly free as long as you have data). Now I'm scared to send her a message in germany. Would it be free from my computer or my iphone or?

Thanks

Added (1). Forgot to add that for my imessage on my macbook, I have both my email and my number selected for me to receive messages at. Should I change this?

It should be free. As long as you're sending the message via iMessage and not SMS you should be fine. If you iMessage via your computer there's no way for your phone company to charge you because you're in your internet without SMS.

To make sure that you both are using iMessage, make sure the conversation between you guys in your Message app is blue and not green. When it is blue that means iMessage is being used and when it's green you're sending SMS.

And it shouldn't matter if you have Wi-Fi or not for your iPhone's. For your Mac, yes you would need Wi-Fi because your data plan is connected to your cell plan, while with a Mac you don't have a cell plan.