Image sizes on large monitors not upscaling

Hi, I scaled my carousel images before uploading to be pretty small so they’d load quickly (640px wide, which is perfect for a 3-image-wide slider on a 1920px monitor). Problem is, they don’t upscale to still fill larger screen sizes and have unappealing large gutters between them. Any easy CSS to tell the carousel to always fill 100%? I’d rather have a slightly fuzzy upscaled image on retina monitors than use larger images that take forever to load. Thanks!

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