This feature is pretty new in the SharePoint Environment and has been Targeted for the Mobile devices. Let's see what Image Rendition does for you.
Using the Image Renditions, you can have the original (large) images stored on your website and then define a number of scaled variants to be used throughout the website. Cool, isn't it?
Let's say you have an Image A (500 *400) and you can create multiple variants for the same image for dimensions like (200*200), (300*200), still maintaining the quality.
A mobile device demands fast retrieval and a limited screen space, here, the Image Rendition comes as a rescue for you!!
To set your own image rendition templates,
Navigate to Site Settings under Look and Feel group and Image Renditions option.