For complete peace of mind, you can safely buy resources from the below sites for photos, vectors, videos and illustrations, as all comes with a royalty-free licence.
*My post contains some affiliate links.
- Big Stock Photo
- Stocksy United
- Haute Chocolate
If you are using an image on anything that you will be making money on (like selling a t-shirt), you MUST have a commercial use license to use that image. Using an image without the right license is a copyright infringement and punishable by law.