Comment Re:Where are these photos? (Score 4, Informative) 336
Not by default at all - you have to specifically add the photos to iPhoto and then turn on iCloud in system preferences.
Downloading pictures off a camera/usb stick/android phone can be done with Image Capture, and this does not put them on iCloud, just into folders on your computer.
Adding them to iPhoto is what puts them onto iCloud, and only if you turn it on - when you set up a Mac for the first time it asks you if you want it switched on (and prompts for an Apple ID).
If you use Aperture or iPhoto you can still keep things local only - there's a checkbox in preferences that turns off the iCloud sync.