Before you use any photos or videos you did not create or own, you should ask the copyright owner for permission by contacting them directly. If you want to use "free-to-use" images, then make sure to click the "Use these Images" link in the Push To Store Fulfillment Supplier section as these images come directly from the supplier and can be used directly on your website.
If you would like to use the videos in advertising campaigns, then make sure to contact the supplier before you do so. They should have no problem with this, since you will be ordering products from them. If you are on the Nexus Push to Store page for that product, you can click on the “Contact” link in the Fulfillment Source area to directly connect with the supplier.