iPhoto has an internal file storage system where it saves all the photos for the events, albums etc.
You notice this, when you select an event and click "Show in folder". Then you see the whole folder with all the pictures in it.
You should treat events or albums as your folders. You can give them custom names and everything.
What I have done is:
- Import all photos to iPhoto
- Delete original files
- Just work with iPhoto (when I want to see the photos in a folder structure, I can choose "Show in folder".
Note: I have heard that you can have an external file storage for your photos with Apperture. This is iPhoto with more functionality for 200 USD, in case you do a lot of work with iPhoto this might be interesting.
Photostream will store the previous 30 days worth of photos automatically. That means, every photo you take on your iOS devices will be sent to Photostream, but only the last 30 days worth of photos will be available there.
If you enable 'Automatic Import' of Photostream in iPhoto (Preferences > Photostream > Automatic Import) then when you launch iPhoto, it will automatically download a copy of any photos that are in your Photostream, and automatically create an Event titled: {MMM YYYY} Photo Stream (eg. "Jul 2012 Photo Stream")
This Event will be automatically kept up to date, as long as iPhoto is open long enough to sync everything required (depends on your connection).
In effect, as long as you open iPhoto once every 30 days and leave it open long enough for your Photostream to be imported, you should never have to manually import a photo from your iOS device at all.
As for removing items from your Camera Roll on your iOS device, as long as you see that they are present in Photostream (or in iPhoto), then it is safe to delete them from your device. (There is no way around this but to do it manually as far as I know.)
Currently, Photostream does not support videos. To import videos, connect your iOS device to iPhoto and import these manually.
As a side note, in order to keep some photos on the device, you can create a smart album in iPhoto along the lines of "Photos taken within the last 30 days" or "Photos rated 4 stars or more", and configure iTunes to sync these to your device. That way, you can be sure that no matter if you clear your Camera Roll on your device, or delete certain photos, you maintain some convenience.
Best Answer
I was just on the phone with Apple tier-two support, and I asked the same question.
The agent's response was yes, both the iPhoto app and the iPhoto library can be safely deleted, if you're confident you've migrated all media to the newer Photos app.
I've done so, and I've seen no trouble on my Mac nor iPhone.