If you go to Edit->Preferences, you will get a popup of upir general options. The first tab has a field on the lower right labelled: Max Cached Renders. This is the total number of renders that will be saved by Poser. For example, mine is set to 10. After 10 renders, the oldest ones get overwritten with each subsequent render. I found where they are stored once, but there really is no need to delete them manually. If it bothers you that much, just set the Max Cached Renders low, and that should solve that problem.