Responsive Image Formatter Tools

Drupal Core does provide a good foundation for defining responsive images in a website. However, implementing a proper solution requires a lot of configuration to work in tandem,

For a given "responsive set", you need following information to design a proper responsive solution

• Available Breakpoints
• Pixel Density Multipliers that needs to be supported (normal screens, retina screens, high DPI displays...)
• Image Styles based on Image sizes that re needed
• Responsive Image Style which contains mapping of image sizes with breakpoints

Depending on the relative size of the picture that you are displaying with respect to the viewport, you would need more than one responsive set in a website. 

RIFT module provides you an opinionated but yet very powerful and flexible way to tackle this problem space.