<?php
$directory = '/path/to/directory';
$directoryIterator = new RecursiveDirectoryIterator($directory, RecursiveDirectoryIterator::SKIP_DOTS);
$iteratorIterator = new RecursiveIteratorIterator($directoryIterator);

$files = [];
$fileInfoResource = new finfo(FILEINFO_MIME_TYPE);
/**
 * @var SplFileObject $fileObject
 */
foreach ($iteratorIterator as $fileObject) {
    if($fileObject->isDir()){
        continue;
    }
    $mimeType = $fileInfoResource->file($fileObject->getRealPath());

    if(strpos($mimeType,'image') !== false){
       $files[]=$fileObject->getRealPath();
    }

}

var_dump($files);