You probably realize that adding visual elements drastically improves the overall user experience on a website. 

You can’t, however, snag images off the internet. As the owner of a website it is your responsibility to not violate copyright, and to determine if and how you can use an image. Violating copyright can unfortunately come with some pretty hefty financial consequences so it's important to realize that every photo (from a google image search to a Facebook post) gains copyright as soon as it's created.
When in doubt:

• Use your own images!
  If you are the photographer, there's no harm in violating any copyright. The biggest bonus: authentic images of you and your products or services always communicates an authentic and real depiction of your brand. Hiring a professional to take images for your website is also a great idea depending on your budget.
• Purchase licensed stock photos. 
  Creators license out their work for purchase. Once you purchase a commercially licensed image or video, you then have the legal right to use the file in any way you wish. The advantage is a legal, high quality photo. A disadvantage to consider is the obviousness of a stock image not being "yours" or representing the uniqueness of your mission or business vision.