I was puzzled about this myself quite recently so did some searching on the Internet. When I drag and drop from iPhoto into a folder it seems to remove it from the former, which is OK if that is what the user wants. However, if you want to keep a copy in iPhoto, the way that I found to do it is to highlight the picture within iPhoto, go to File > Reveal in Finder > Original. You can then right click and copy and paste into your chosen folder. Mind you, that is a lot of work for hundreds of photographs.