Use the correct path to the images directory.

The `is_dir` and `open_dir` methods in PHP look for a directory relative to the .php file being ran. The previous code was looking for a "Medium" directory in the same directory as "Marts.php", but the directory did not exist. The correct directory name should be "images".
Use the correct path for images and the image links.
Exclude directories in the file list.

The method readdir returns all entries in a directory, including entries that
are directories. Instead of only excluding the '.' and '..' entries, the code
now only returns entries that are files.
Merge pull request #1 from barberboy/master

Fix the incorrect paths to the image files and the images directory.