Why do you hate having naked pics of ex-girlfriends re-appear on your new computer?
Serious answer, iPhoto is merely a "place holder" for your photos, the same as iTunes is for your song files. I believe the actual picture files are held in a folder somewhere inside the system folder.
Perhaps you somehow still had pictures on your old computer in your downloads or somewhere else on the drive? In that case, iPhoto simply found everything with a .jpg .jpeg or whatever picture extension available on the drive and "imported" them in.