The annoying thing is that this is supposed to be a time-saving convenience feature. If you're using the same texture repeatedly, you don't have to keep browsing to its folder. But the developers neglected to include a remove feature. So we're stuck with clutter. Sucks, doesn't it? Just another item on kupa's to-do list for Poser 5. The only way to flush the unused textures is to shut down and restart Poser.