That's an error if you are using an old version of SMF Gallery Lite which did not have those text strings or using a language pack where that string does not exist.
Now i have installed new version but i have the same error with gallery_text_oldpicture
butr is very sample to repair,
simply open /forum/Themes/default/Gallery.template.php, search
echo '<div align="center"><br /><b>' . $txt['gallery_text_oldpicture'] . '</b><br />
and replace with
echo '<div align="center"><br />
Now all work fine!
PS i have found other small bug in template now i have fix and my gallery is valid W3C
Thank'you very much for your work,
PS sorry for my bad english