How to find images?

Question The question is often advisable to add images to his articles. This contributes to the success of some blogs. But how to find a picture to illustrate each page?
We want not to purchase images whose origin is not known ....

Search for images

The first source is the search engine Google Images.

"The most comprehensive image search on the Web."

The Advanced Search interface offers a range of options to refine your search, including options about copyrights and right to reuse.
This may be defined in the query too, based on the following parameters:

Images in the public domain:

&as_rights=cc_publicdomain

Images CCA licensed (Creative Commons Attribution):

&as_rights=cc_attribute

Images CCASA licensed (Creative Commons Attribution Share-Alike):

&as_rights=cc_sharealike

So if you're looking for royalty-free images about "programming", the request will be:

http://images.google.com/images?q=programming&as_rights=cc_publicdomain

In a group keywords are separated by the %20 code, example:

programming%20php

Sites offering images for free

Some sites provide lots of images and icons available for free...

A list of resources on Wikipedia.
Commons. On the site itself.
Wikiwix. The images tags allows to search free images on Wikipedia.
Burningwell.org. Repository for public domain images.

Icons

Another list for icons.
Pixuff. Free icons.
FamFamFam. Icons for applications.