Your image names are correctly formatted, so this could be a program issue (bad / corrupted PHP files somewhere).

Have you installed any other modules? Some can overwrite the additional_images.php file(s).

If you have followed the template over-rides procedure correctly AND you are aware of any Add-On over-writes that have occurred, you could try loading a fresh set of zencart files to the server, to REPLACE the current core files on the server.

BEFORE you do this, make a full backup or copy of ALL your folders and files that relate to your ZC installation.