how to find image only links in string and linkify them

You can add another argument called $allowed_types, which holds all the extensions you want to allow.

Then you must get the substring after last '.' character, and compare it to your list of allowed extensions.

This is the basic idea, I'm sure it can be improved alot.

/**
 * Turn all URLs in clickable links.
 * 
 * @param string $value
 * @param array  $protocols  http/https, ftp, mail, twitter
 * @param array  $attributes
 * @param string $mode       normal or all
 * @return string
 */
function linkify($value, $allowed_types = array('jpg', 'png'), $protocols = array('http', 'mail'), array $attributes = array()) {

    /**
     * Get position of last dot in string
     */
    $dot_pos = strrpos($value, '.');
    if(!$dot_pos) {
        return FALSE;
    }

    /**
     * Get substring after last dot
     */
    $extension = substr($value, $dot_pos + 1);

    if(!in_array($extension, $allowed_types)) {
        /**
         * Extension not in allowed types
         */
        return FALSE;
    }


    // Link attributes
    $attr = '';
    foreach ($attributes as $key => $val) {
        $attr = ' ' . $key . '="' . htmlentities($val) . '"';
    }

    $links = array();

    // Extract existing links and tags
    $value = preg_replace_callback('~(<a .*?>.*?</a>|<.*?>)~i', function ($match) use (&$links) {
        return '<' . array_push($links, $match[1]) . '>';
    }, $value);

    // Extract text links for each protocol
    foreach ((array) $protocols as $protocol) {
        switch ($protocol) {
            case 'http':
            case 'https': $value = preg_replace_callback('~(?:(https?)://([^\s<]+)|(www\.[^\s<]+?\.[^\s<]+))(?<![\.,:])~i', function ($match) use ($protocol, &$links, $attr) {
                    if ($match[1])
                        $protocol = $match[1];
                    $link = $match[2] ? : $match[3];
                    return '<' . array_push($links, "<a $attr href=\"$protocol://$link\">$link</a>") . '>';
                }, $value);
                break;
            case 'mail': $value = preg_replace_callback('~([^\s<]+?@[^\s<]+?\.[^\s<]+)(?<![\.,:])~', function ($match) use (&$links, $attr) {
                    return '<' . array_push($links, "<a $attr href=\"mailto:{$match[1]}\">{$match[1]}</a>") . '>';
                }, $value);
                break;
            case 'twitter': $value = preg_replace_callback('~(?<!\w)[@#](\w++)~', function ($match) use (&$links, $attr) {
                    return '<' . array_push($links, "<a $attr href=\"https://twitter.com/" . ($match[0][0] == '@' ? '' : 'search/%23') . $match[1] . "\">{$match[0]}</a>") . '>';
                }, $value);
                break;
            default: $value = preg_replace_callback('~' . preg_quote($protocol, '~') . '://([^\s<]+?)(?<![\.,:])~i', function ($match) use ($protocol, &$links, $attr) {
                    return '<' . array_push($links, "<a $attr href=\"$protocol://{$match[1]}\">{$match[1]}</a>") . '>';
                }, $value);
                break;
        }
    }

    // Insert all link
    return preg_replace_callback('/<(\d+)>/', function ($match) use (&$links) {
        return $links[$match[1] - 1];
    }, $value);
}