See these links:
Are you asking about iPhoto on your mobile devices or iPhoto on your Mac? And if Mac, what is your iPhoto and Mac OS X version?
If you enable Photo Stream in one of your Photo Libraries on your Mac, you can decide, if you want to have automatic import or export from the photo Stream.
If you select automatic import, all photos from the stream will automatically imported in monthly events, for example "November Photo Stream".
And if you select "Automatic Upload" any photo you import to iPhoto will be added to the Photo Stream. But you can opt for manual import and upload. Then only the images you drag from the PhotoStream will be added to your library and only what you drag to the Photo Stream will be uploaded to your IOS devices.