Headliner

<spoiler> The small pictures of Houdini along the bottom are actually signals representing letters. The code/language of Semaphore is used, among other things, for nautical signaling. A quick Googling for images of Semaphore will yield a primer. Thus, the message along the bottom reads:
OPPOSITE
The skywriting clue from Australia was DARK.
Thus, the answer is: LIGHT </spoiler>