I had the same question as macinchik: What is thumbnail navigation? This is a configuration option, there's a button for it in the System Options panel of the Admin interface. It would be helpful if this configuration option was briefly explained on the Configuration page of the Documentation: http://plogger.org/docs/config/
Anyway I edited the picture.php file in the folders for each of the 3 themes, by adding print plogger_get_thumbnail_nav(); as in ddejong's instructions.
In the Classic theme the thumbnails showed up below the photo I was viewing, but the thumbnails were arranged in a vertical column, which made the page too long. In the Air theme the thumbnails were arranged in horizontal rows, but the same thumbnails were repeated again below the footer. In the Lucid theme the thumbnails were arranged vertically below the photo, on the left, except for the first thumbnail which was on the right and was overlapping the border. In all the themes the thumbnails did function as navigation.
So now I get what thumbnail navigation is. A nice option, but maybe not ready to roll out just yet.
EDIT: I restored the picture.php files to their original state. Now in the Air theme, the thumbnails are showing in one horizontal row below the photo. The thumbnails don't show in the other themes. So there must be code in the Air theme to show them, and when the extra line of code was added, extra thumbnails showed. But there must not be code for thumbnail nav in the other themes. Which is why some people like macinchik and myself wonder what it's all about. If you've got Classic or Lucid activated, you click on Thumbnail Navigation Enabled and nothing changes.
You are right, jordan, the Classic and Lucid themes don't have the neccessary line to output the thumbnail nav. They are also missing any styles for the thumbnail nav in gallery.css, so if you did add the plogger_get_thumbnail_nav code, you'd still need to style the nav.
Air is ready to be used with thumbnail navigation. You may want to use that theme to see how Plogger outputs the nav, and perhaps look at gallery.css to see one way to style it. (BTW, the most recent version of Air is at http://www.ardamis.com/2007/07/05/plogger-beta-3-theme-air/.)
I saw mention of being able to limit the amount of thumbnails shown, but no solution posted and the link in that particular post is now inactive.
Is there a known (documented) way to be able to do that? Let's say, have five thumbs with a prev/next option to see more?
edit: I changed the Thumbnail Navigation Range to '3' in the admin options and this works nicely to limit the amount shown. Still curious about a prev/next option though.