Show only picture from post_content in custom RSS file

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you can use regular expression on your post content.Which could be done for example like this:

for reference

$content = get_the_content();
// we need a expression to match things
$regex = '/src="https://wordpress.stackexchange.com/questions/190375/([^"]*)"https://wordpress.stackexchange.com/";
// we want all matches
preg_match_all( $regex, $content, $matches );
// reversing the matches array
$matches = array_reverse($matches);
echo '<pre>';
// we've reversed the array, so index 0 returns the result
print_r($matches[0]);
echo '</pre>';