Below are some free online options for finding reusable media.
As a student, all sources must be credited in your work - even if they're public domain or CC0 licensed.
Try the Intellectual Property database to look up an artist before you use their work commercially: https://wiki.gettyimages.com/
Google has added an easy-to-access limiter for searching through Google Images for items labelled available for reuse.
However, you can only be 100% sure you are looking at the actual usage rights if you get the image from a legitimate source. This can be more difficult to ensure. Is the source credibly the copyright holder of this image?