Using the built in Simplepie library in Wordpress, this function fetches the first img of each post. Display it using: echo '<img src="' .get_first_image_url($item->get_content()). '"/>';

simplepie-fetch-image.php

/**
 * Gets RSS Image Item Link
 *
 * Place the following inside rss markup
 * to display the the first image of that post:
 * echo '<img src="' .get_first_image_url($item->get_content()). '"/>';
 *
 */
function get_first_image_url($html) {
	if (preg_match('/<img.+?src="(.+?)"/', $html, $matches)) {
		return $matches[1];
	}
	
	else return 'url_of_default_image.jpg';
}

Why do you use Simplepie for it?

I was using it to fetch featured image from a feed I didn't own, which means I couldn't change the output and the feed wasn't getting the featured images (so I couldn't use get_featured_image). So I ended up with this solution where I took the first image of each post instead and used that as my featured image.

Is there easier ways of doing this?