I've setup a motion tracking scene with trackers, background etc.. I have 2 objects in my scene: a cube that renders without problem, and an imported rig. That imported rig does not appear in the render, but its shadow is present. I have checked the layers, the texture.. Thank you for your help :)
I've finally solved the problem ! Thank you for your answer anyway. It was because an object was in Blender Cycles, I've just converted it into Blender Internal and it worked like a charm.