Stock photography is when a photographer uploads images to a website with the intent of selling a license to use the images. There are 2 main types of stock. Royalty Free - pay once and use the image in various ways, but have no control over who else is using the image at the same time - and Rights Managed - Pay for each specific use of the image, but you can arrange exclusive use. There are advantages to both depending on what the image is needed for. Royalty Free is best when an image is needed for several applications (ie when an article will be posted both in a blog and in a printed newletter) Rights Managed is better for editorial or new related work where the buyer wants to be the only person publishing the image at the time. (ie exclusively on "News Blog")
There are a number of stock sites available and pricing ranges from a few dollars per image to several hundred. Examples of stock agencies include Shutterstock, Dreamstime and Alamy. By using stock images you will often get better quality options for the theme or topic you need. You also avoid some of the copyright issues as long as you follow the licensing agreement of the site.
Hiring a photographer, or taking the images yourself, is the third option. This option is best if the images need to be of specific items, like a craft blog explaining how to make a quilt or a food blog post with a specific recipe. Depending on the quality of the photographer's work the images could be striking and beautiful. The downside is that hiring a professional can also be expensive. Most will charge $100 or more per session, not including travel time. The use of the images will only be limited by the contract between the photographer and the client.
Finding images to spice up a social media posting can be easy if you know where to look and understand the restrictions. Where ever you choose to get the images from please, don't just steal them.