It's always a good ideal to protect yourself when using images you didn't create. The following a few website I've run across that have good options. You'll have to check the terms on each and setup accounts. Here's a nice article on copyright and using images, "The Ultimate Student Guide to Images".
Some stock vector websites:
These are a few services that offer stock photos for free or a reasonable price. Deposit Photos tends to be my go to website for stock photos.