Free images links

A good site where to find good public domain images is among others publicdomainpictures.net
Most of these pictures are directly usable, others ask you to send a cup of coffee to the author or other dummy actions :)
Note that below are sponsored iStockPhoto images that are not free, as clearly stated.

Another good site is Stock-xchng
You may search in the search box or browse the categories (+ image categories)
Low res images may be saved from the page, to download hi res images you need to register (warning: do you want to receive their newsletter and ads? if not, uncheck!)
Images can't be used for immoral or illegal content or redistribution or blablah but normal use is completely free.