The N64 uses some funny formats, relies on transparency, visible images are dependant on render mode settings in graphics display lists, and not everything you see is even textured or uses a texture pre-se. I don't know off-hand how many textures in Mario are compressed, but it, like virtually everything on the platform, uses compression. I believe there are global textures just like there are global objects, but individual levels embed used textures (and other resources) within them.
There is an editor for Mario 64 to automate image viewing and extracting. You may want to try using it first before poking around directly.http://qubedstudios.rustedlogic.net/ToadsTool64.htm