If you look at the JSON you are pulling in, there is a
href key, that contains a URL. That URL goes to something like this:
https://colorlib.com/wp/wp-json/wp/v2/media/36414 where you have a
rendered key that contains the image that you need.
What we would need to do is to parse the original JSON payload, then fetch each image URL, then fetch each image, which would be possible, but seems like a weird way to do things.
Maybe there’s a better way, but I’m not overfly familiar with the WordPress API and I’d need to spend some time reading around it.
Also, can I ask why you are taking this approach? It’s not actually an RSS feed we’re dealing with here, rather exposed API endpoints on a site that you don’t own. If you pull in all of their images and make a slider of them, it’s probably copyright infringement.