There is a simple way of adding textures to the program. Click the paint bucket icon in the menu bar, and a new window will open called Materials. Click the Plus sign icon that's labeled Create Material. You will see the Texture field. Browse to where the texture is saved on your hard drive. Once it's been added, rename the texture and adjust its size. Click OK and it should be added to the Model collection of textures.
You can download additional textures using the websites listed through Google Search: https://goo.gl/DzSwcq