Enabling the Render banner images @2x retina toggle option will automatically export any JPG and PNG images at retina (@2x) resolution. This will give you sharper looking images in your banners, but will also increase the file sizes of any JPG and PNG images.

You can also optionally override this per layer by clicking any layer's Settings icon button in the main timeline to open the layer settings, where you can specify forcing the layer to be exported as 1x, 2x, or left as the default (which will use the global Render banner images @2x retina toggle option, which is the default).