I hope you realize that your motors will only be powered by 4.3V or so, even though your source is 12V. The PIC will put out a maximum of 5V from its output pins, and those drive the bases of the two 'top' transistors in the bridge. The emitters will go no higher than a diode drop below the bases. If you really are using the circuit you show, your 'top' transistors are going to dissipate a lot of power, and your motor won't have much power.