Some people use Google images - can be risky.

Other than that look for some WSO's that have hundreds of royalty free images.

Check out public domain images. Here's a list of a few sites:

Public Domain Pictures - Free Stock Photos
Public Domain Images
Free Photos - Free Images - Free Stock Photos - FreeDigitalPhotos.net
stock.xchng - the leading free stock photography site
Flickr: Creative Commons

Make sure you read their terms before using any images. Some require attribution (giving credit to the original source) in order to use them, and others don’t allow commercial use.

or you can edit a little bit the image. if you change at least 3 elements from the picture, is consider a new picture. but ofcourse, be carefull what images are you editing

I would advise buy at istock, I know it cost money but at the end of the day, you are having peace of mind...

There are images that are royalty free. Make sure you read its terms of use to play safe. Good luck.

depends on the niche you are.

Check what your competitors are doing.

Stock photos are highly suggested.

If you are talking large volumes, then use an image scraper, and bulk editor to slightly change the pitches like adding a watermark. Okay so this is black hat, but it works for large volumes although can get your site in trouble