Whenever you use images, be sure to follow their license. While it is common for people to Google an idea and grab images from that search to use on your site, except in very rare cases, that violates copyright law and you can be sued or required to pay retroactively for use of those images. While that’s less likely to happen for a social media post or something ephemeral you post just for yourself (unless you have a lot of followers or deep pockets) it is pretty likely to happen if you build a site for a business or post something for a business and use unlicensed and copyrighted images, because a business typically has deeper pockets than an individual and a lawsuit may be more worthwhile. There are many sources for licensable images, some even available for free, but don’t use an image unless the license says you can and if the license has requirements like attribution, be sure you attribute it properly and do whatever else might be required.