[cellarius]

So you're not getting a db-error and the script is pulling information from the database; else you'd probably see a mysql-error or the result would be empty. Most likely there's something wrong with the path to data-dir where your photos are stored.

A possibility to check this would be:
Look at the source code of the popup window. Look for
Edit: By that I meant the html-source in your browser, not the files of the mod. Sorry for misunderstandigs.

onclick="thumbpaste(

The code should read something like:

onclick="thumbpaste('<a href=\'http://www.yoursite.com/path/to/gallery/datadir/category_number/filename.jpg\'><img src=\'http://www.yoursite.com/path/to/gallery/datadir/category_number/thumbs/filename.jpg\' style=\'border: 0\' /></a>'

Please look at the part:

<img src=\'http://www.yoursite.com/path/to/gallery/datadir/category_number/thumbs/filename.jpg\'

Is the URL to the image thumb valid and does it correspond correctly to your gallery installation? You can check this easily by trying the URL in your browser window.

If there is an error there, we can look to resolve it. If you want to, you can tell me your gallery settings (paths to gallery root and datadir) and copy the relevant part of the popup's source code here.