Here what was my idea: Nanoha and Homura in the foreground, back to back, with weapons out and Walpurgisnacht in the background

Nanoha Takamachi (I'm using 9-years-old nanoha as reference, as otherwise she would look much older than Homura):
https://encrypted-tbn0.google.com/images?q=tbn:ANd9GcQM6DM6PnZRTEvxsu3MHwa3Q4hNeC2pMIB6AbXB5TT3HQUkoGoTbg
Pose and weaponry reference:
https://encrypted-tbn0.google.com/images?q=tbn:ANd9GcQIioRbyvGaJmLLG4x5EwS075pp3IiOXy9mADZFYKqkI4-J0q

Homura Akemi:

Pose and weaponry reference:
http://images.puella-magi.net/thumb/1/1b/HomuraFigma10.jpg/525px-HomuraFigma10.jpg?20120120131404

Walpurgisnacht background:


Mind that there should be enough image under waist level of the two girls in order to put this logo without covering important part of the image (their faces and their weapons, mainly)
https://lh4.googleusercontent.com/IdndsTPFKY7jAB5rLZLEyIOSngW4053GfkMKtsasIcO67-pJV9Chl36Qtmw6aQVJsfc0pnizQCwxxWrhD5CRJnZCLUQqJ2mBNtc9qJWg4gcsP3JO-I0

If you think that following these guideline is too complex, you're free to change whatever doesn't work. Enough google-fu should provide you with enough images of both characters.