client/lib/post-normalizer/rule-content-detect-media.js
function isCandidateForContentImage( image ) {
	if ( ! image || ! image.getAttribute( 'src' ) ) {
		return false;
	}

	const ineligibleCandidateUrlParts = [ 'gravatar.com', '/wpcom-smileys/' ];

	const imageUrl = image.getAttribute( 'src' );

	const imageShouldBeExcludedFromCandidacy = some( ineligibleCandidateUrlParts, urlPart =>
		includes( imageUrl.toLowerCase(), urlPart )
	);

	return ! ( isTrackingPixel( image ) || imageShouldBeExcludedFromCandidacy );
}
function isCandidateForContentImage( image ) {
	if ( ! image || ! image.getAttribute( 'src' ) ) {
		return false;
	}

	const ineligibleCandidateUrlParts = [ 'gravatar.com', '/wpcom-smileys/' ];

	const imageUrl = image.getAttribute( 'src' );

	const imageShouldBeExcludedFromCandidacy = some( ineligibleCandidateUrlParts, urlPart =>
		includes( imageUrl.toLowerCase(), urlPart )
	);

	return ! ( isTrackingPixel( image ) || imageShouldBeExcludedFromCandidacy );
}
Returns true if image should be considered
@param {Node} image - DOM node for an image
@returns {boolean} true/false depending on if it should be included as a potential featured image