The first time you start the Google+ app, it will ask you if you want to enable Instant Upload (which uploads to a private album from which you can publish). Prior to that it doesn't do anything with your photos.
I have them syncing with DropBox intentionally.
It asks you if you want the uploads to take place when you first setup the app.
I may not be correct about 'any' android phone though. I've only used stock and several custom ROM on Galaxy Nexus ranging from version 4.0.4 to 4.2.2. After you add a Google/gapps account you'll see this in sync setting- http://s24.postimg.org/fxbv98s05/Screenshot_2013_04_28_23_42... . I've found 'Google Photos' always checked by default. First time this feature was introduced, I didn't notice and my G+ filled up with random images from my mobile gallery. Since then I consciously turn this off everytime I flash a ROM.
Edit: so you don't necessarily need Google+ app installed for this to happen.
I don't really mind though. Good backup.
That's the same way malware/toolbars get installed...
1. buy new note2 from att, register google account, skip samsung and att setup.
2. buy new s3 from amazon unlocked, register google account, skip samsung setup.
3. never even open G+ app on both phones
4. take a picture
5. wait an hour
6. you get a notification "pictures you took are ready for sharing" i.e. they are already uploaded against your will and out of your knowledge.
If someone complain about a bad Android default or behavior in a non nexus device, and you have a nexus device, just stay put!
Nexus are a completely different beat when it comes to user control, ok?
Not only does it come pre-installed but you also can't uninstall it.
Admittedly it's been a long time since I tried, perhaps they fixed it since then.