Sure, you can use Google Images, but those aren’t always copyright free — and you should use only copyright free images or images for which you’ve purchased copyrights.
Morguefile – The search works fairly well, but the image quality is very uneven. There’s a lot of bad stuff (bad composition, terrible lighting, no creativity, etc.), but there’s also the occasional gem.
Wikimedia Commons – This collection leans heavily toward historical, technical and scientific collections. For example, you can find drawings by Van Gogh, maps of cities, or anatomical plates of the human body. (Art nerd heaven!)
Here’s a post listing four other sites (in addition to Photo Pin) that are good sources for free image.