Lost photos from iPhoto library - where are they??

I can't find any of my old photos from the "iPhoto" app. I do have all the ones in the "Photos" library. HELP!! A lot of pics that can't be replaced! I don't see a "Pictures" folder.

I have bought a new computer and have had several OS changes since the iPhoto/Photo changeover.

I am not terribly computer literate so please help in a way I can understand. Thanks!