[checkoutemail] Some Thumbnails in order email don't preview correctly

I've noticed that some thumbnails that are included in the email that is generated with an order do not preview (irrespective of the mail client/software).

I've got a feeling that it may relate to the filename length. My galleries all have the file extension stripped out on upload via Gallery Remote. As an example, titles/file names of photos that will correctly preview a thumbnail in the order email are as follows...

SS26001
1004012
V3037001F

Whereas, titles/file names of photos that will NOT preview a thumbnail in the order email are as follows...

Kimmins
MonicaWedding006

Anyone got any idea why?

Gallery version = 2.2.3 core 1.2.0.5
PHP version = 4.4.7 cgi
Webserver = Apache/1.3.33 (Unix)
Database = mysqlt 5.0.45-log, lock.system=flock
Toolkits = ArchiveUpload, Exif, Getid3, LinkItemToolkit, NetPBM, Thumbnail, Gd, jpegtran, SquareThumb
Acceleration = none, none
Operating system = Linux infong 2.4 #1 SMP Thu Jan 13 08:59:31 CET 2005 i686 unknown
Default theme = matrix
gettext = enabled
Locale = en_GB
Browser = Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-US; rv:1.8.1.9) Gecko/20071025 Firefox/2.0.0.9
Rows in GalleryAccessMap table = 109
Rows in GalleryAccessSubscriberMap table = 552
Rows in GalleryUser table = 6
Rows in GalleryItem table = 548
Rows in GalleryAlbumItem table = 12
Rows in GalleryCacheMap table = 0