We have the option to download all renditions together from the assets GUI as seen below,
You could also use the rendition sidebar to select a rendition and download it via right clicking on the image on the right side as seen below,
Or else you can traverse to the file in CRXDE Lite and right click on the node " assetname.jpg/jcr:content/renditions/[desired-rendition] " and click on open. This will download your image rendition to your local.