It may be Poser keeps a 'file lock' on any texture it loads, so it knows when some other application changes the file. I think it must keep the current texture in ram, otherwise it would be pretty slow when using the textured preview (having to go out to disk everytime you change the pose or view. Unless it keeps a low rez version loaded for preview and loads the full texture in for rendering. Might also depend on your Video card's memory as well. Heck if I know.