Correct, there're high res textures ingame, but you don't need them for bsp.
Put the textures to your pball textures folder (.../pball/textures/pball/hr4/). Note that this have nothing to do with bsp.
Example
If you have the (low res) texture ...pball/textures/pball/grass1_8.jpg (128x128),
you need the (high res) texture ...pball/textures/pball/hr4/grass1_8.jpg (512x512).