Which wood do I need...

[Wingedharpy]

to make a panel on which I intend to put a poster/picture then screw this to a wood door?
I don't want it to warp.
I am confused with the choice of panels on offer.
Help!

A thick bit of hardwood or ply, perhaps?

I would use plywood or mdf for this.

use mdf say 7mm thick painted both sides to stabilise timber, emulsion or gloss your choice

Thanks to all.
It gives me a starting point.