The shape of Spain's production possibilities frontier (PPF) should reflect the fact that as Spain produces more digital cameras and fewer camcorders, the opportunity cost of producing each additional digital camera .

Remain constant.

Explanation:

As Spain produces more digital cameras and fewer camcorders, the opportunity cost of producing each additional digital camera remain constant.

Production possibility frontier is a curve that show how different combination of product are produced using limited resources. It demonstrate that how production of one goods need to be decreased to produce higher number of other goods.  

Opportunity cost is constant, as tradeoffs are the same regardless of where you are on the line, same slope at any point.