Netsuite doesn't have anything native for this but a simple-ish solution would be to create a node app on AWS Lambda that accepts an image and resizes it.
You can't deploy a user event script on images so you'd be stuck with a scheduled script or a map reduce script. You can feed that with a list of images created since some date/time or you could set a User Note when the image has been resized and filter against that.
I've used the node package 'sharp' for this sort of thing and the speed and quality are good.