The most expensive mistake is pulling images from a search engine. Image search is a finding tool, not a licensing tool, and the overwhelming majority of what it returns is protected by copyright. A specific industry exists around this: rights holders and their agents run automated scans that match images across the web and issue demand letters for past unlicensed use, frequently for sums far above what a license would have cost. Those letters arrive years later, they cite images you may have forgotten adding, and the fact that a designer or a former contractor put the image there does not transfer responsibility away from your business.
The licenses you will encounter fall into a few categories worth recognizing. Public domain material has no copyright restrictions at all. Creative Commons Zero is effectively the same in practice. Other Creative Commons licenses attach conditions: some require you to credit the creator, some prohibit commercial use, and some require that anything you build from the image carry the same license. Royalty free means you pay once and can use the image repeatedly under stated terms, which is not the same as free. Rights managed means the license covers a specific use, for a specific time, in a specific place, and anything beyond that needs a new license.
Two additional restrictions catch people even when the license is otherwise permissive. Images containing recognizable people generally require a model release before they can be used commercially, and a permissive license does not by itself grant that. Trademarks and identifiable private property in an image carry their own limits, which is why an otherwise free photo of a busy street can be a problem in an advertisement. Most free sites address this in their terms, and most users never read that far.
Protect yourself with a habit rather than a policy document. Keep a simple record for every image you use: where it came from, the date, and a copy of the license terms as they read when you downloaded them, because site terms change and your defense is the version that applied at the time. Download from a small number of sources you have actually vetted rather than whatever a search surfaces. And when a contractor supplies images, ask in writing where they came from and what license applies, because their sourcing becomes your exposure the moment it goes on your site.