Your website is the central pillar to your online marketing and with this in mind, you should consider using bespoke images to enable the best website design. Hire a photographer and get original, top quality images to represent your brand.
If time and budget don't allow this, then you may decide to use one of the options below:
If you are on a very low budget, and don't mind working with a limited subset of images, then there are some great sites offering royalty free images. Some sites don't even require attribution.
Sometime, you can't find the perfect image on any of the free resources. When this happens, you might like to consider searching through one of the paid-for stock image sights.
And there are plenty of other sites that a quick google search will pick up.