The only application I have on my Mac that works with PhotoStream is iPhoto. I'm wondering if my Mac grabs the images from PhotoStream constantly or if it only downloads them when I launch an application that uses PhotoStream (such as iPhoto).
Yes! A process called
PhotoStreamAgent runs at login, and idles in the background, downloading new photos when they're uploaded to your Photo Stream.
There exist both
/Applications/iPhoto.app/Contents/Library/LoginItems/PhotoStreamAgent.app, indicating that this background-downloading behavior works no matter which application you use to manage your photo library.
(And as Asmus points out, photos are stored in
The photos are "constantly" downloaded to your Mac, and stored in subfolders within