How to find free images?

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 ....

Searching for images

The first source is the search engine Google Images.

"The most comprehensive image search on the Web."

Google has created options for rights in its image search engine.
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:

Filters

&as_rights=cc_publicdomain
&as_rights=cc_attribute
&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 

Going directly to sharing sites

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.
• Vector buttons and images. qVector is a sharing site for vector elements.

Icons

• Another list for icons from Commons.
• Pixuff. Free icons and images.
• The noun project. Free icon of any thing.
• Vector desktop objects. Big icons.

Images generator

• Robohash.org generates images of robots from a text. Based on identicon.

See also

• List of royalty-free icons for web pages and applications.