So, you're planning on renting a car in Spain, you don't have a photocard and the guidance says that you might need one (legally).
It doesn't matter whether or not you legally need an IDP to drive in Spain if the rental car company won't rent you a car without one.
Effectively, your question is a compound question consisting of whether you legally need an IDP and whether or not the as yet undetermined hire car company will rent you a car without one.
I say wing it. If you get there and can't get a car, it doesn't affect us.