Residual button files how to delete

Hello,

   I am trying to use the captivate feature which allows for having rollover buttons by using the file naming scheme xxx_up  xxx_down  xxx_over. It works great, but everytime I want to change the button out, I have to use a different name than before (in the first part, before the underscore), or the old button keeps coming up. How do I stop this from happening?

Thanks,

Dan