iPhoto stores pics chronologically. To see how it's done, follow this path, home folder------>pictures---------->iPhoto Library.
In your iPhoto library, there are folders labeled by years. Within them, there are folders for each month. Within those, folders for each day.
If pics are taken with a digital camera, they automatically have a time stamp. If pics are scanned, they go in the folder corresponding to the date they are scanned.
I don't know if you can have pics arranged any other way. Others here will know probably.
Hope this helps.