a lake with mountains and a house reflected in it

This can be anything "offline" that's not on the web, e.g.: Books, clothing, posters, birthday cards, advertising etc.

Insert the attribution line: "Image from: Wallpapers.com." or "This has been designed using content from Wallpapers.com" close to the image you're using. If this isn't possible, then place it in the credits section.

Offline or Online Video platforms (Youtube, Vimeo, Dailymotion, etc.)

Paste the link below with the accompanied text in the video description area: This content derives from Wallpapers.com

For example: Apps, games, desktop apps, etc.

Paste the link below on the website where your app is available for download or in the descriptionfield of the platform or marketplace you're using to post your app, games or desktop apps.