Royalty free photos
www.flicker.com: Flickr is a Yahoo service. A great share of the photos on Flickr isn’t royalty free, but you can limit your search to royalty free photos. Click on Advanced Search below the search field and check Only search within Creative Commons-licensed content in the Create Commons area.
www.google.com: Google indexes all photos on the internet, whether they are royalty free or not. After performing a search query, you can click on the gear button and pick advanced search. There you can choose to only show royalty free pictures (free to use or share).
www.everystockphoto.com: You can make a free account here and find photos.
This article form TheNextWeb offers an overview of a large number of websites offering free photos.
High quality, paid photos
www.istockfoto.com: Create an account if you regularly need photos from this site. You can buy credits using a credit card. Bigger photos generally cost more credits to purchase.