com.provenance.n64/hires_texture/{ROM NAME}
The ROM NAME isn't the file name, but instead the identifier for the ROM in the header of the ROM file. If you don't know what it is, you can view the console output at load where it says something like, Mupen (3): Name: SUPER MARIO 64
So in this example, if I wanted to load hi res textures for Mario 64 I would put them in, com.provenance.n64/hires_texture/SUPER MARIO 64