There are many places to find photographs online. Some of them are protected by copyright, others are public domain, and others use varying levels of creative commons licenses that sometimes do or don't allow reproduction. As always evaluate your sources and be aware of the rights usage for images you find online. Below are a list of some helpful online photography collections.
Depending on your topic, you may want to move beyond the normal search engine image search.
Keep in mind that search engines such as Google only scratch the surface of what is available online. What cannot be easily found with a search engine is often called the invisible web. For example, databases serve as gatekeepers that may prevent the web from crawling it. Also, depending on the algorithm used for searching, useful images might not rise to the top.
Examples for evaluating sources: