I dont what your question is exactly .. but i know from experience that placing those transistors the wrong way you will easily cook the circuilt ... why are you building a H-brigde from descrete components anyways ... i would suggest you get the LM293D H Bridge IC. it is the exact same thing as the circuit you show exept it is capable of controlling two motors.
If you put pnp at bottom and npn at top (and appropriately swap there biasing through opto-couplers),
1) Just think what would be the voltages at the motor terminals during operation.
2) Think what would happen to your switch resistances. (Do they act like a switch and would they be able to operately easily in the required regions).
weeb0,
PNP transistors are used in the "top" of a bridge, so that they can be driven without the use of a separate power supply for the drive. If NPNs were used on top, then they would have to be used in the emitter follower configuration which necessitates a supply voltage more positive thatn the bridge power. Also, you would always have a Vbe drop inherent in the emitter follower configuration.
Regards,
Kral