Quick Start

Create a directory on your computer.
Copy some photos inside this directory, resize them for web display. Warning: the name of directories and files must contain only letters, figures, "-", "_" or ".". No blank space, nor accentuated characters.
With a FTP client, copy your directory in the "galleries" directory of your Piwigo installation.
Logon your gallery and go to Administration » Pictures » Thumbnails to create missing thumbnails.
Go to Administration and click on the big Synchronization button.

Congratulations! You have successfully created the first album of your photo gallery.

Directories and files organization

Album directories are in the Piwigo directory "galleries". Here follows the directory tree of a very small gallery (but using many features):

.
|-- admin
|-- doc
|-- galleries
|   |-- category-1
|   |   |-- category-1.1
|   |   |   |-- category-1.1.1
|   |   |   |   |-- category-1.1.1.1
|   |   |   |   |   |-- pwg_high
|   |   |   |   |   |   +-- wedding.jpg
|   |   |   |   |   |-- thumbnail
|   |   |   |   |   |   +-- TN-wedding.jpg
|   |   |   |   |   +-- wedding.jpg
|   |   |   |   +-- category-1.1.1.2
|   |   |   +-- category-1.1.2
|   |   |-- category-1.2
|   |   |   |-- pookie.jpg
|   |   |   +-- thumbnail
|   |   |       +-- TN-pookie.jpg
|   |   +-- category-1.3
|   +-- category-2
|       |-- piglet.gif
|       |-- pwg_representative
|       |   +-- video.jpg
|       |-- thumbnail
|       |   +-- TN-piglet.jpg
|       +-- video.avi
|-- include
|-- install
|-- language
|-- template
+-- tool

Except for those named "thumbnail", "pwg-high" or "pwg_representative" (see explanations below), each directory under "galleries" produces a category. There is no limit on deepness.
Basically, an element is represented by a file. A file can be a Piwigo element if its extension is among file_ext configuration parameter possibilities (see include/config_default.inc.php file). A file can be a picture if its extension is among picture_ext configuration parameter possibilities.
Picture elements must have an associated thumbnail (see section below about thumbnails)
Picture elements can have a high quality file associated. As for wedding.jpg in the example above. No prefix on the high quality picture.
Non picture elements (video, sounds, file texts, whatever you want...) are represented by default by an icon corresponding to the filename extension. Optionally, you can associate a thumbnail and a representative file (see video.avi in the example above).
Warning: the name of directories and files must contain only letters, figures, "-", "_" or ".". No blank space, nor accentuated characters.
Once files, thumbnails and representatives are correctly placed in the directories, go to: Administration » Categories » Synchronize

Thumbnails

Each photo must have its thumbnail.
Thumbnails are stored in the "thumbnail" sub-directory of the category directory. The thumbnail has the same filename as the corresponding photo, with configuration parameter prefix_thumbnail as prefix (see include/config.inc.php).
Thumbnails files have not necessary the same extension as the associated image (eg a ".jpg" image may have a ".GIF" extension).
It is recommended to use an external module for thumbnails creation.
The usage of the thumbnails creation tool integrated in Piwigo is not advisable, because the resulting quality might be disappointing and this will lie heavily on the server ressources (and that could be a serious problem on a shared server).
If you choose to use the webserver to generate thumbnails, you must allow write access on all category directories.