That's right, using some time-traveling technology that the ILM boys whipped up (c'mon they invented Photoshop like 19 years ago, what do you think they've been doing in the meantime?) I traveled all over the globe planting these 13 skulls.
Actually, I only planted 9 of them, Jar Jar kept dropping them. 3 broke, and one of them landed in a giant pile of poodoo. Don't worry, I belted him soundly in the snout with my trusty ForceFX lightsaber.
Why ILM can build a fully-functional time machine, and not get me an actual working lightsaber, I'll never understand. Gotta get the boys on that.