If I have a charge +q at a distance d above an infinite non conducting plane, why should the image charge at d below the non conducting plane be +q?
as the plane is non conducting, the lines that you draw for the electric field should not stop on the plane. therefore a line that starts from the charge, then goes towards the plane, should at some point be deviated and at very large distances be almost parallel to the plane.
if you would put a negative charge for the image charge, then remove the plane, the electric field will join the two charges, hence crossing the plane.
to avoid this, you have to put a charge with the same sign, the lines you draw have to repel each other like the plane would do.