。
She laid her head on the pillow.
Literal
She [topic-は] head [object-を] pillow [on-に] placed.
のせた (載せた/乗せた) means 'placed on top of' — a concrete, physical verb. 枕にのせた is the natural way to describe laying one's head on a pillow in Japanese. Simple and direct, with no grammar complications beyond basic を and に particle usage.