You can put an image for a channel, like a logo for the feed. But the items on the feed doesn’t has a tag for images.
For my usage, I’ve used the ‘flog’ namespace for my fotolog feed and be able to syndicate my fotolog at my WP blog.
You can see it at https://tenak.net/blog , and read about the format of the feed at https://flogfeed.tenak.net/
But as VaamYob says you before, the reader has to know about the URL of an image on the feed to show it.