Key Terminology: "Copyright Free Images?"
Very few photos are copyright free, only PD photos have no copyright attached. In most all other cases the original photographer still owns the copyright over the image because they created it. That does not mean that you can't use the image, but in order to use any image you need a license to do so. In the case of free and creative commons images the license is given away with no charge, in the case of paid "stock photography" then you need to pay for the license depending on what use you make of the image. When you buy a stock photo you are not buying the copyright of it, you are simply buying a license to use the image.
A stock photo is simply an "off the shelf" image that was taken with the intention of being reused in a design, web site etc, this compared to having someone take a custom photo for you. Stock photos can be obtained either free or by purchase. The term stock photo is used to differentiate these images from those which were taken as artworks, or for documentary or editorial purposes.
Public domain images, images that have been made available for free use and have no terms attached, typically the choice of subjects and image resolution or quality is limited - there's no such thing as a free lunch!
The terms which you agree to when you 'buy' an image for use, these typically prohibit you from reselling the image or using it in certain predefined ways, as stated above then you 'buy' an image you are in fact buying a license to use it, you are not buying the copyright of the image, a common misunderstanding amongst amateur image buyers. Licenses vary from site to site so always check you can use the image in the way you intend. There are two main license types in stock photography Rights Managed (RM) and Royalty Free (RF) More about stock photo license types...
A system of licenses which allows photographers to provide their work for free under certain well defined conditions, these sometimes exclude the image being used in commercial applications or being modified and ALWAYS stipulate that the photographer be attributed where the pictures are used. Many photographers use creative commons as a way of promoting their work in front of a wider audience.
CC0 (Creative Commons Zero)
A special Creative Commons license that allows photographers to waive all their rights (PD or "public domain" dedication), unlike all other Creative commons license types there is no need to attribute the source of a CC0 image, and indeed the image can be sold, redistributed of built into a commercial product provided that other "Rights holders" are not affected (see below).
A term applied to Royalty free stock photo agencies who source their images from not just professional photographers but also amateurs via a strictly controlled review process. Microstock sites are a marketplace for 1000's of photographers to sell their work to you, but buyers only need to deal with the agency itself. Most sites allow you to buy credits that afford you a number of downloads of varying resolutions, most also sell vector illustrations and some sell stock footage (video).
Strictly speaking the term refers to how many pixels (the smallest elements of a digital image) are displayed or printed in a defined area (i.e. 300 dots per inch - 300dpi) but the term is often used to refer to the dimensions of an image in pixels e.g. 1200x1600 pixels. The higher the number of pixels the larger you will be able to print or display the image without losing quality.
A rights holder can be one or more of:
Additional rights holders are the reason that most commercial images do not feature recognizable brands, or if they do those images are marked as 'editorial only'. While you can for example use a public domain photo for any commercial use you like, you would likely find yourself in trouble if you used a public domain image that featured a celebrity to endorse your product without their permission. There would be no problem from the photographer or a copyright holder (none for a PD image), the problem arises from what is featured within the image and the context it is used in).
You pay a monthly or annual fee to access a pre defined number of photos as you need them, this works out cheaper if you need images on a regular basis.
This term is usually applied to images of events or celebrities, they have special license terms applied to them, they normally cannot be used in advertising or promotional material.
A term used by stock agencies to allow images to be used in more applications than normally allowed e.g. to resell a photo as a derivative product like a framed print, website template, poster etc. then you need to buy an extended license or merchandise license, these cost more than standard royalty free licenses.
Royalty Free (RF)
Stock photos which are supplied for unlimited use (within the license terms which do sometimes include restrictions) are called royalty free because you do not pay a royalty each time the image is used or how many times it is displayed. Microstock images are sold as RF.
Rights Managed (RM)
Rights managed is usually the most expensive way to buy stock photos, but has some key benefits in exclusivity, as rights managed images are strictly controlled, you pay to use them on a per use basis e.g. on the front of 10,000 copies of a magazine, or on the front page of a web site for 1 year. RM images are much less common these days, RF is more flexible for image users.
Attribution / Credit Link
An attribution or credit is a piece of text or web link displayed along with an image when you use it, unless used in editorial applications paid stock images almost never require a credit. Conversely a large majority of free stock photo licenses stipulate some kind of 'credit' as do all creative commons licenses; in this case you can think of the attribution being made in lieu of payment to the photographer. If you use the images without making the credit then you will be breaking the terms of the license leaving you open to legal action - a pitfall a lot of people fall into when the simply download a photo they see on the internet and use it.