Right - last import is exactly that - the last import so each time you do an import you have a new last import
and if the photos are or ever were in last import then they are in your library
However you posted in the iPhoto for IOS forum but state you are using iPhoto '08 on OS X 10.9.5 so it is hard to be sure what you have
as a general statement if you are not finding photos in you library then yo are looking in the wrong place - one common error is a bad time/date setting on your camera so you are not looking in the correct event for them photos
Usually (unless the date is really off) you can see them in the last "n" months album - or before doing a new import look in Last Import - right click on one and show event to see where they are - normally once you see where they are the problem and solution will be obvious
LN