how to use Data URI for images in WP?

Data URIs don’t cache. So you have the same overhead each page request instead of allowing the user to cache those image requests. For product and banner images it doesn’t make much sense to take extra processing power to convert them on the fly (each request) or pre-process them which means they’ll be stored in the database. It’s possible… but I wouldn’t say I can recall anyone ever doing it that way with WP.