Options.
- Wait for webflow to implement it. I only know one CMS where I can do this on client upload now.
- Create three image fields (or four) for different break points and have the client upload optimized correctly sized images. Then assign the right one as the background image source. Look at Kraken.io
- Stop using background images for images.
- Get a pro account on Cloudlfare, turn on Mirage.
- Offer optimizing service to client and handle it yourself, make some extra cash.
- Try using GitHub - levymetal/jquery-resize-image-to-parent: jQuery script that crops an img to its parent container.
- fill in blank here
These are the ones i can think of off the top of my head.
Or live with a terrible user experience where huge images choke the mobile user’s browser and sucking up their data plan for no good reason.