Let's talk about websites where you can find reference for your rendering projects. Obviously, You can go to Google and find lots of reference there. But there are three problems with reference you find on the Internet. And those are copyright, resolution, and quality. The first is Copyright. If you use someone else's copyrighted material without asking permission, you can get sued. The chances of you getting sued are slight if you massively change the photos you're using, color correcting and distorting them, or only using small pieces.
However the chance is still there if you're using copyrighted material. Second is resolution. I think every matte artist has had the experience of finding what looks like the perfect piece of photo reference, but when you click on it, you find it's not at a usable resolution. The higher resolution you start from with your reference material, the sharper it'll be. And the more distortion and color correction it can stand. The last is quality. Most of the photos on the Internet have severe JPEG compression on them, to make them as small as possible.
Those compression artifacts compromise the image to a greater or lesser extent. So finding higher quality reference is desirable. That brings me to the first great site for finding reference for you rendering, environment-textures.com. It costs a bit more than $100 a year to belong, or about $18 a month for monthly membership. You can download three photos a day for free if you register with them. The site was set up specifically for matte and texture artists and features a huge collection of photos of every possible subject.
It doesn't have people on it, the same organization has another website specifically for figures. But it has buildings, mountains, skies, everything you could need for a rendering, all at very high resolution and extremely good quality. If you're a yearly member, you can also download large quantities of photos of the same subject in a zip file. I've been a member for many years, and would highly recommend you buy a membership, once you become a working professional.
Also, there's an effort made to shoot the photo straight on. In general, having photos with no perspective in them is easier. If the photo was shot with the subject at an angle, you'll often have to remove the existing perspective before you can distort it into the perspective required for your project. With a membership in this site, you can use the photos for commercial projects, as long as you modify them and use them creatively. One thing you can't do is download a bunch of the photos and resell them unaltered. Next is freetextures.3dtotal.com.
This site doesn't have anywhere near as many textures as enviroment-textures.com, but like the name says its free. All the photos are at very high resolution and good quality. After that, its cgtextures.com. This site has an usual policy where you can download up to 15 megabytes a day. And then it cuts you off for 24 hours. You can also buy a membership for around $80 a year, and you can download up to 100 megabytes every 24 hours.
Once again, these are very high quality and at a usable resolution. The next site I'm going to mention is the most expensive one, cgskies.com. This is run by the same people who run CG Textures, and it's all about skies. Each sky costs about $30. And these are huge files up to 18,000 pixels wide. The free version that give away so you can test it in your project is 3000 pixels wide. This price may seem steep, but the quality is top notch.
And if you're working on a big project and need the perfect sky at very high res, this is the site to visit.