Image renditions are a new feature of SharePoint 2013 that let you insert multiple sizes of the same image using the same source image. Image renditions specify the height and width to use when adding an image to a page.
Using image renditions requires BLOB cache to be enabled for the SharePoint web application. See http://technet.microsoft.com/en-us/library/cc770229.aspx for more information.
Follow these steps to create an image rendition:
Navigate to the site in your preferred web browser.
Select Site settings from the Settings menu.
Select Image Renditions from the Look and Feel section as shown in the following screenshot:
On the Image Renditions page, select Add New Item.
Provide a name for the image rendition in the Name field.
Provide the height and width for the image rendition in the Height and Width fields as shown in the following screenshot:
Click on Save.