This app is free from the App Store today. It usually costs $.99. It is available for Apple, Android, and Windows devices. It is very simple and intuitive to use, and automatically syncs your notes to all your devices over the Cloud. You can use different colored backgrounds for you notes and add photos to them.
You can learn more about it here: https://itunes.apple.com/us/app/sticky-notes-hd/id543932220?mt=8