Bunch of questions about Core Web Vitals site improvements

Hi all, web developer here. I've been tasked with improving a few sites in regards to Core Web Vitals. I've generated a report with Lighthouse but have a few questions. I'm hoping some one here might be able to shed light on them:

• I have a bunch of 'Appropriately sized image' errors. There are for banner images that shrink/expand relative to the browser window size. How can I address this issue without making an infinite amount of appropriately sized images to load at each potential window size?
• Are CDNs for third part scripts still preferred over locally hosted scripts?
• I'm told to remove unused CSS classes and there are a bunch for the home page (I have just 1 CSS file). Should I be creating individual CSS files for each page type?

Many thanks for the above!