If My Photo Stream is enabled on your iOS device, any new images taken or imported onto your device will be uploaded to My Photo Stream automatically when your device has a WiFi connection. iCloud will store your photos for 30 days so that your devices have time to connect and download the photos.
If you have enabled My Photo Stream in iPhoto, your recent Photo Stream photos will automatically appear in iPhoto. When Automatic Import is turned on, iPhoto will import your Photo Stream images at full-size to your iPhoto library. This option, available in the Photo Stream tab of iPhoto Preferences, allows you to permanently save all your Photo Stream photos in your iPhoto library.
If you connect your iOS device to manually import photos into iPhoto, iPhoto will automatically detect which photos it has already imported from Photo Stream and display these photos in the "Already Imported" section of the import window. Selecting the default "Import Photos" button will import only photos that have not already been imported from Photo Stream.
If you would like to re-import photos that iPhoto has already imported from My Photo Stream, you can select the images in the import window and then click the "Import Selected" button. iPhoto will prompt you to resolve duplicates.