The best option for any project in which you might need photography is to hire a professional photographer. By doing so you will have the best quality material and you can control exactly what you need to photograph and how it is portrayed. Professional Photographers are not cheap and that is due to multiple reasons. First of all, they have had to study and master their art, and in so doing they have had to invest money and time to perfect their craft. Also, there is the cost of the equipment. Most professional photographers have very expensive equipment and must carry it around with them to be able to achieve the quality of work that they desire and you deserve. Then there is the ever-changing panorama of digital photography that makes them have to constantly upgrade and change things around to keep up to date.
Nevertheless, it is definitely by far the best choice if your topic requires it and your budget permits it.
Royalty Managed Photographs
This is a cheaper option than the first but it is still rather expensive. With Royalty Managed Photography you are provided by the company that sells the photo the history of the image. You can also purchase images that have the release of important models, which would otherwise be impossible to afford. With this type of photos, you must provide the company with a lot of information as to the usage of the material: type of piece, industry, time of usage. This is to be able to guarantee that the image you are purchasing was not used two years ago by your competitor in billboard advertising or in some other industry for which your company might have ethics issues with such as smoking or child control.
Most of these companies are very rigorous in their screening process but this is to protect all involved.
These are great photographs for general non-specific things. Their cost is relatively low and their quality is very good. If you need a generic photo of a nice beach that no one will really be able to know where it was taken, this is the way to go. But, beware, since they are very good images you could find that you are using the same corporate meeting photo in your brochure as your competitor. It is an affordable option but precisely because of this you should be very careful where you use them and if you are using them for a client you should always advise them of it so they are aware of the possible risks.
Free Stock Photographs
There are a lot of very good free photographs online. They are great for blog posts and social media inspirational quotes but beware of using them in the corporate setting. The quality of these photos is not necessarily consistent. Some may be very small files that will not give you the final print size you need and others are just plain out of focus. Nevertheless, you can find very good material that with careful use may help you in presentations or newsletters.