Morgan Kay

If you use a service such as Feedburner or MailChimp to turn your RSS feed into an email subscription, and the images in your posts are more than 600px wide, the formatting of your HTML emails might be messed up. This plugin lets you resize the images in your RSS feeds.
Version 2.0 has been completely re-written from scratch! It is now more reliable, and works with images from external URLs.

RSS Image Resize

Resize images in RSS feeds so they will fit in HTML emails.

simsketch

If you need to add the post’s featured image as a Open Graph meta tag to the RSS feed, ie: for Apple News.

Featured Image Meta Tag in RSS

If you need to add the post's featured image as a Open Graph meta tag to the RSS feed, ie: for Apple News.

RSS Image Resize vs Featured Image Meta Tag in RSS