Although drawing and painting "from life" is way much more effective and efficient than drawing from photos (I'll be writting a separate journal about this soon, no worries), you simply can't have a person to model for you every time you want to create something. And there are situations when you don't exactly know how to place that bloody hand or if the foot is drawn correctly. Similiar issues can happen with architecture, perspective... with everything you draw. Then, when you look for help, you may stumble upon Stock gallery and you find awesome things. Remember about crediting the used stock directly!
POSES AND ANATOMY
ARCHITECTURE
ANIMALS
OBJECTS