note that a large portion of the wikimedia commons is licenced under some terms requiring at least attribution. Make sure you're checking the licence before you use images from the commons.
(not saying you shouldn't use wikimedia commons, it's a great resource. just don't use it blindly)
There are also many, many gems among the 20+ million other free images at https://commons.wikimedia.org.
The site has an extensive category system that complements keyword-based search, e.g. https://commons.wikimedia.org/wiki/Category:Fruit.