Please try the following:
1. While in Composer mode first resize and position the head so it is correctly placed.
2. Then select the Head image so it has a blue bounding box around it.
3. Now click the "Attach" button from the top toolbar and click on the Head Bone to attach the head to the body.
4. Return to Stage mode and you should find the head will move with the body when it is animated.
Note: If you are not sure which bone is which in Composer mode, just mouse over the bones and the name of the bone will be displayed.