My two favorite sources for images are depositphotos.com and dollarphotoclub.com.
I like Depositphotos best because it doesn't require an ongoing monthly subscription. I average less than 60 cents an image. Partly, This is because I usually buy a 1 month subscription and download 5 images a day. Also, once a year, I've been getting a "special" offer from AppSumo.com ($39 for 100 images from Depositphotos with no expiration date).
While Dollarphotoclub (Curated by FOTOLIA) requires a minimum $10 monthly subscription; they are worth considering because they have quality images; many of which I don't see elsewhere.
When I have a larger budget I also like GraphicRiver.com part of the Envato Marketplace.
- The Pragmatic Web Designer.
Royalty free is the search term (Free to use) as a filter.
For paid I agree with Daniel Rusling
You need to look at the stock sites - a special database of images. You can also buy ready-made templates presentations, diagrams and graphics for your website on http://www.pptstar.com/
You might try Getty Images. It is a pay site but you'll get better quality images if you pay for them. Good luck!
Getty Images now has a program where you can use their excellent images without charge. They continue to host the images on their servers and provide you with linking information for accessing their images,
Free images for commercial use
I would highly recommend 123rf or Getty images.
https://creativemarket.com/ is a great resource for illustrations.