A free license is a license that is designed to ensure the freedom of a work (such as a text or an image). Free licenses are typically copyleft licenses, i.e. they allow anyone to use the work, for any purpose, as long as they follow the terms of the license, which include the condition that the same freedoms must be granted and be explicitly included for all copies and all derived work. This ensures that a work that was once released under a free license will remain free forever: nobody can ever make it unfree again.
"Freedom" here means free as in "free speech", not necessarily free as in "free beer". Anyone can use free-licensed works commercially if they want to (just like anyone can use public domain works commercially if they want to), for example in a printed book or on an Internet paysite.
Examples of free licenses are:
The background of free licenses is that all creative work of authors, photographers, artists etc. is by default copyrighted and may never be used by others without explicit permission from the copyright holder. According to current law, in most countries this copyright expires 70 years after the creator's death. At this point, the work enters the public domain and can be used freely by anyone for any purpose. But before this point, any creative work is by default copyrighted and thus unfree.
A copyright holder who wants his/her work to be freely useable before the normal copyright expiration time can either explicitly release the work to the public domain (i.e. release all rights), or can release it under a free license. The difference between these two choices is that a free license can a) demand that anyone who uses the image must give credit to the original creator, b) ensure that all derived work will also remain free. Both a) and b) are not ensured when a work is released unconditionally to the public domain. Therefore, a free license is often the preferred choice.
How to search online for free licensed images
You can go to Google Image search (see link below) and enter your search keyword(s) or phrases. On the results page, click on "Search tools" and select "Usage rights" -> "Labeled for reuse with modification".
You can also search for images that have a Creative Commons license that allows them to both be modified and used commercially via the Creative Commons Google Image Search Assistant (see link below). It also can perform simultaneously the same search on Flickr.