Not enought? Useful third parties resources:DeviantART - Varied tutorials and resources made by users.
Pixabay - High quality images, illustrations and vector graphics.
Textures.com - Digital pictures of all sorts of materials.
Poliigon - Materials and HDR's for artists who want photorealism.
Texture Haven - Textures licenced as CC0 with direct download.
Solar System Scope - Equirectangular textures based on NASA imagery data.
NormalMap-Online - Normal map online generator.
SoundBible.com - Free and royalty free sound effects and clips.