We abide by copyright laws and you may not use images/content from someone else without their permission. If you think that the material you are using may belong to someone, there are a few things you should do:
1- The best thing is to reach out to them and ask for permission to publish.
2- Give credit to content/image owner
3- If you quote or paraphrase give credit to the source
4- Simply not use, find alternatives. ( e.g.: for images or videos do a search for stock images)