The "All Posts" page - /wp-admin/edit.php - does not show a featured image just by using WP Core. You have to add code to display featured images there. You may need to look back at your backups to see what code you were previously using to get the featured image to show up.
It could be that you edited your theme's functions.php file, and when you updated ...
Since you are using ACF, you should use a repeater field. This will give you a nice admin interface for adding/editing/removing images. Once you have the fields created and added to a page you just loop through the data with PHP, here is where you would create the markup needed for owl carousel.
Here is an example of getting the data from a repeater.
You were very close. You just had a single-quote swapped with a double-quote after the onmouseover.
<img src="https://website.com/wp-content/uploads/2019/08/image1.png" onmouseover="this.src='https://website.com//wp-content/uploads/2019/08/image2.png';" onmouseout="this.src='https://website.com/wp-content/...
@Ooker It's the Jetpack that is serving you the images on https. If you'd turn off the Photon add-on from Jetpack you should be able to get images served over http from your own server.
Also if you turn off Photon, images won't be served from CDN anymore, so you might have to figure that out.