RSS - Really Simple Syndication

I use Inoreader and I've noticed that the RSS feed for lemmy.ml has proper thumbnails/previews for images while the lemmy.world feed doesn't show any images (just links for the posted images). I contacted Inoreader to see if the problem was on their end. I received the following feedback:

Actually, we've already checked that, but unfortunately, we couldn't improve the image preview for the exmapled feeds. The thing is that the provided links to these images are not properly presented in the feed source, which is causing that. We suggest you address this with the feed publishers.

What settings need to be changed so that the images are "properly presented"?