Primary Rule of iPhoto - iPhoto is a SQL database program and you must NEVER make any changes of any sort to the structure or content of the iPhoto library using the finder or any other program except iPhoto!
By default (and very stongly recommended) iPhoto copies imported photos into the iPhoto library and you can delete the source from outside the iPhoto library. this is called default "managed" library (the iPhoto preference to "copy imported items to the iPhoto library is in its checked state). When you delete a photo from an event in iPhoto, empty the iPhotio trash and empty the system trash then that photo and everything about it is gone form your system. iPhoto never touches any photo outside of its library so you must manage any photos that you have anywhere else.