Because RSS feeds are generally not standardized and can be generated by anyone, there is no specific format. We will do our best to pull the top image from them (either through an explicit URL tag, or by pulling the images tags in the actual post body) but it will not always work. If it doesn't, our best suggestion is to make sure the RSS formatting as closely matches the standard as possible.
A good example of an RSS feed is https://www.wired.com/feed/rss.
If you have any questions or need any help, please email us at firstname.lastname@example.org