Happens EVERYWHERE you go.
Most of these images you will never know about.
One in 100 you will know about.
Experts have stated that no less that seven times a day every citizen will have a picture taken - without consent.
However, regarding pictures of homes listed by realtor :) , it is with the permission of the homeowner at that time. Sometimes a rogue individual will steal the image taken by an agent and place a fraudulent 'For Rent" ad on Craigslist. Images real estate professionals or RealTors take or pay to be taken are syndicated to websites many of which the agent has no knowledge.
Then, there are the county taxing authorities who take infrared and aerial images of homes. These are publicly viewable. EVERYONE can see if you are grilling hots or burgers in the back yard. Then there is the water authorities who use aerial imaginary to enforce irrigation restrictions. And of course code enforcement loves the access to all the imagery.
Then of course there are those new smart phones that automatically post images with geo tags to YouTube and Facebook. To be quite frank, there is no place to hide where one can avoid 'you people' from taking your picture without your permission. Some will argue that this promotes SAFETY. Others may argue it promotes CRIME. The truth is, it is here to stay.
If you're talking about photos from the MLS, those were taken, at some point, with the permission of the home owner. Sometimes--though it's not always best practice--if a current photo isn't available but the house was listed, say, a few years ago, that picture probably is still in the MLS system and can be used to populate the current listing. (Not always best practice since the appearance of the property may have changed in the interim.)
No one from Trulia (that I'm aware of, at least) is skulking around neighborhoods with a camera.
Hope that helps.
I'm Chad Brown. I'm own a brokerage in Traverse City and also serve on The MLS Committee and was on The Board of Directors for The Traverse Area Association of Realtors. Your question sparked my interest, but I'm not exactly sure what you're referring to. Can you be more specific?