The codes are in two parts for each postbit that you want to control the images in. Without the postbit code it won't work. The CSS you've posted is for vBadvanced CMPS. That code is strictly for the Portal's news section.
For postbit_legacy, in your Admin CP go to:
Styles & Templates --> Style Manager --> All Style Options --> Edit Options --> Postbit Templates --> postbit_legacy
If you still need help let me know and I'll help you through it.