Usually, the Google image search tool "labeled for reuse" should do the trick. Quite often, it will show you images on pixabay.com.
In general: if you can't see a licence at an image, better not use it. The best pictures for usage are the ones with PD (Public Domain - no copyrights) or CC0 (Creative Commons licensed - the creator has set zero terms on their use). There are some others you can use, but this is usually explained next to them.