Sigue las siguientes instrucciones. Si no entiendes alguna terminología técnica puedes usar Google para informarte.

In the administration, go to the batch manager and select an album : once you have validated the filter, some thumbnails should appear :

• if you get a “No photo in the current set.”, the upload failed : be sure you didn't add too large a picture, according to page Photos→Add “20MB. jpeg, png, gif.” for example.
• if you see “XX photos in current set” but just empty squares, read paragraph 2
• if you see “XX photos in current set” and the thumbnails, read paragraph 3

Since 2.4, Piwigo has an automatic generation system for thumbnails and other resized pictures. Piwigo calls i.php to generate the resized picture in ./_data/i/, then directly calls the resized picture from ./_data/i/. Many problem may appear : an overloading of the server, a picture that is too big or with a wrong extension, not enough permission on files/folders etc

1. Check if an error is returned by the i.php url : in the Batch Manager, right-click on a failed picture then “Display the picture” or “copy the url to the picture”, from your browser. You can also try to display the source code of the page (Ctrl+U usually) and search (Ctrl+F) for an url with “i.php”. Then go to this url with your browser and note any error message displayed.
   1. “source not found” : check by Ftp, if the source picture exists and if the permissions of the file is enough (Chmod : 755 folders, 644 files). If it's still not solved, be sure the Php user/system has enough rights on your server
   2. “dir create error” : check by Ftp, if the permissions of the folders are set right (Chmod : 755 folders, 644 files). If it's still not solved, be sure the Php user/system has enough rights on your server
   3. “Empty array while parsing Sizing” “Sizing arr” “Invalid chars in request”… : the theme or plugin is not working, post on the forum with a list of the plugin used and your theme
   4. 404 error page : the file i.php is missing. Download the zip from piwigo.org, extract and upload the files by Ftp. Overwrite all the files : you will not lose any customization or pictures.
   5. 403 error page : check the permission on the file i.php (and the other files also). Chmod : 755 folders, 644 files.
   6. “500 Internal error page” or “PHP Fatal Error: Allowed memory size of …” : many problems may cause this error. If some pictures are generated and others not, it might be due to an overload of your server (just wait a few hours) or a lack of memeory for PHP (ask to your hoster to change the memory_limit option). If the graphical library is GD (first admin page), install Imagemagick [see below the paragraph related to Imagemagick]
2. No message?
   1. With Localfiles Editor, put this local config variable : $conf['enable_i_log'] = true; . Then try to generate some size with the Batch Manager. Once started, you can go to _data/tmp/i.log and open the file to see if there is any error message in it. Post on the forum the content of that file
   2. You can also read the instructions for “500 Internal error page” and use Imagemagick. Or post a url to your website with public visible pictures Thanks

Most cases are permissions issues :

1. Check the level of privacy : in the Batch Manager, click on Edit for a problematic picture. Check the level of privacy, under “Who can see this photo?”. Then go to User→Manage→Properties column and be sure the level of privacy of the users match the one set for the picture. On the Batch Manager, if the picture has a level of privacy different from “Everyone”, it's written right on the thumbnails.
2. Check the visibility of the albums : Go to Album→Properties, and check if the albums are private or not. If they are private, and you did it by mistake, revert them back to public. If not, go to Album→Manage then browse to the album, and then click Edit. Or go to the album on the public part, and click the edit icon. Once you are the edition page, click on the Permission tab : you can see exactly what you see for that album, by clicking on “Show Details”. Enable the plugin Multi View, the easiest and fastest way to verify Who Can See What : it opens a pop-up and then select a user in the drop-down list

ImageMagick es mucho más eficiente que la librería gráfica GD. Menos consumo de recursos en el servidor y imágenes de mayor calidad. Muchos problema pueden resolverse cambiando la biblioteca gráfica de GD a IM, (además tu proveedor de alojamiento estará encantado).

Utiliza las siguientes variables con LocalFiles Editor:

/* Biblioteca utiliza para redimensionar imágenes. Los valores pueden ser 'auto', 'imagick',
   'ext_imagick' o 'gd'. Si el valor es 'auto', la librería se escogerá en el orden
   mencionado. Si una librería no estuviera disponible, se seleccionará a la siguiente. */
$conf['graphics_library'] = 'ext_imagick';
 
// Si la librería usada se encuentra en una instalación externa de ImageMagick ('ext_imagick'),
// puedes establecer el directorio ImageMagick.
$conf['ext_imagick_dir'] = '/usr/local/bin/convert'; // en linux

Puedes consultar la librería que estás utilizando desde la página principal del menú de administración:

Pregúntale a tu proveedor de hosting la ruta donde se encuentren los binarios de ImageMagick.

Consultar la información de ImageMagick en la wiki.

Si nada de todo esto ha podido ayudarte, publica un nuevo tema en el foro. Describe educadamente lo que has hecho, añade la url de tu galería y obtendrás la ayuda que necesites por parte de la comunidad.