Use 'Background Image Fit' to make your images responsive. Make your background image span the entire width or height of the frame.
It's best practice to treat 'Cover' is the default setting that automatically adjusts your image to stretch across the full width of a Frame.
'Cover' is most frequently used in banners to create a 'full bleed' image that sits nicely in the background. However, you may use this tool in any frame, e.g. in columns, where you would like to show full width background images.
'Contain' is a neat effect that always presents your image at it true dimensions. Therefore, it's bets practice to think of 'Contain' as a way to show the Full height of the image, where 'Cover' will display the full width.
For designers, some alternate use cases to consider are: