When you hit the render button poser will load all the textures into memory that you've added to your scene, including the ones you decided not to use in that render. What makes things difficult is if you delete a texture & poser still has it in it's list of stuff to load at render time, it'll throw a fit if the texture isn't where you told it to load it from. The best way round this until CL creates a way to unload the textures is to save the scene, close & restart poser then render, that way only the textures you want will be loaded.