How to create thumbnail feed in rss with the post's image

I’m creating a rss feed for my own website with html and javascript, and I want to make a small feed from the news I post on my website. I have access to the item/title/link to source/description/publish-date in my rss. But I also want to add something like <thumbnail></thumbnail> tag with the image src inside.

I’ve used a lot of plugins, and yes some add the post’s image in the rss but then as an <img =”…”> in the description tag or other tag. That’s not what I want and I don’t know how to call the image like that on my own feed that i’m creating with html en javascript. Isn’t there a plugin that just simply create a tag with the image inside?

I’ve tried to costumize the feed-rss.php files in /wp-includes. But nothing happened..