By default, the primary file of a repository object is used when a user views the object. However, any rendition can be used as the default rendition. For example, for a large, high resolution image file, a smaller, low-resolution image can be used as the default rendition.
Only users with at least Read permissions for the file can select a default rendition.
To set the default rendition:
Navigate to the file.
Select View > Renditions.
Select the rendition you want to set as the default rendition.
Right-click and select Set as Preview from the context menu.