public static List<File> getValidFiles(File f){
        File[] files = f.listFiles();
        List<File> fileList = new ArrayList<File>();
        if (files != null){
            for (File file : files){
                if (file.isDirectory()){
                    fileList.add(file);
                    continue;
                }
                if (getFileExtention(file) == null){
                    continue;
                }
                if (getFileExtention(file).equalsIgnoreCase("PNG")){
                    fileList.add(file);
                }
            }
        }
        return fileList;
    }