Take a photo using Siri voice activation? I'm a new user of Siri since I recently upgraded to iPhone 6s. What I can't figure out though is if I somehow can take a photo using only Siri?

Obviously I tried and failed and my google skills doesn't find any information on the subject.

All I can accomplish is to get Siri to launch the camera app, which requires me to enter my passcode, and that is far worse than simply accessing the camera from the lock screen. It seems I'm missing something here.

My set Siri language is Swedish (if it differs?)
At present, Siri is unable to actually capture a photo on your iOS device, regardless of language. This feature can be suggested to Apple via their general Feedback form. 

Saying "take a picture" will, as you mentioned, open the stock Camera app in Photo mode. This is the extent of Apple's functionality for this command right now.