There are iStockphoto, Getty Image, Corbis, Fotolia, etc... Great pictures, but you have to pay to use them, even if it's only to practise Photoshop. You also have Google images, but the pictures are not always aesthetic, and above all most of them have a copyright.

Internet it's the best resource for everything you might look for. Looking for Photoshop brushes? You got it. Cool pictures? Sure! What about textures? That's also easy!