To start with, I will explain a little about matte painting. Matte painting is where you use a texture to paint an image to make it look realistic. This is an example:
I took two royalty-free textures from textures.com and incorporated them into the drawing. I still have to shade, highlight, and create the perspective to get it to work in the image. The textures are on the ground— the rocky ground. The other is on the wall. It gives the image a realistic feel even though not everything is texturized.
Another recent example:
The texture I used on this Holiday Caticorn page on the third panel was also from textures.com It is a seamless texture of one square. I took the texture and copied it continuously until I had the kitchen floor you see. Textures are very useful.
I learned Matte Painting from one of my Computer Graphics courses at IUPUI.
A good book for this subject is Digital Texturing & Painting by Owen Demers.
Another great book for this subject and Digital Painting in general or even general painting is from the master James Gurney Imaginative Realism.
The important thing to remember when you are searching for these textures is that they explicitly say they are Free for commercial use (you can sell the image you incorporated the texture into with no worries of someone suing you because you used their texture) and No attribution required (you don't have to give credit usually, but you cannot claim the original texture as your own— the final image is, but not the original texture file). It is a site by site differentiation and no— you cannot just Google whatever you want and incorporate it into your drawing. Follow the rules, so you are good to go.
Reference is not copying. When you compile reference, you can't just say— oh I like this piece from this character and this piece from this other character and combine the two and make that drawing your own— that's plagiarism. The easiest way to circumvent all issues when using reference— first draw what you can do RIGHT now without reference before you look at anything. This makes sure that the drawing you come up with is your own. Second, compile all images— a lot of images. Separate them from copyrighted material and non copyrighted material. Look and study the copyrighted material without drawing anything and then leave up the non copyrighted material. You want to learn, but you don't want to infringe on copyrights. No staring at copyright material unless they're in a how-to-draw book— only when you are learning to draw should copyright material be your reference, but any character you come up with— won't be your own.
Learning to draw is not the same as using reference. When I was learning to draw— I traced Pokemon characters— knowing that they weren't my characters. When one of my sisters was learning to draw, she exclusively traced everything. It made her good at drawing VERY quickly, but the poses, characters, and drawings she made were not her own mind. I moved on from tracing faster than she did and just looked at the drawing and tried to draw what I saw. This helped me to draw from my mind faster than her, but she was never satisfied with her drawings. I say this because copyright material has its place. It is not to be used as a reference for your own characters. It is exclusively for learning purposes.