Drawing with white chalk on black paper can look really effective and dramatic. But handled badly, it can also look really 'wrong'! The biggest problem is the temptation to outline in white, where we would normally be using a graphite pencil. The trick is to reverse the way we look at things. With graphite or ink, we draw the darks and shadows, and sometimes don't give the highlights much attention. When drawing with white on black, we need to pay lots of attention to the lights, as well as shading the darker and mid tones lightly to bring them into the correct value range. It's a fantastic mental exercise, even if you prefer to work on white paper, as it will challenge you to really think about what you are drawing and develop your sensitivity to light and shade. If you are a pastelist, working with pure white on black is a useful way to improve your awareness of value without the complication of color.
Try the White Chalk on Black Paper drawing exercise.
It's generally better to draw from life than to draw from a photograph, but sometimes having a subject sit for hours isn't possible. Kids are particularly challenging sitters - they just won't sit still for a moment! If you're going to draw from a photograph, you really need it to be a good one. A well-lit picture with a natural subject is so much easier to draw and will give you really great results. It's particularly important when 
If you love fantasy, action and adventure movies you've probably already seen 'Thor' and 'The Avengers' starring the Aussie actor Chris Hemsworth. The movies are based on the Marvel comic books and so don't spend much time in the ancient world - Thor looks like a thoroughly modern hero! Here's a more traditional comic book take on an heroic Viking character for you to draw.