The link and instructions for attribution is provided on every page where you download our image. Just click on How to attribute?
This is an universal attribution for all our images with following code:
Image by <a href="https://www.vectorportal.com">Vectorportal.com</a> by Vectorportal.com</a>
Public domain (CC0)
For images in public domain (CC0) attribution is not required but is appreciated.