Here is the Piwigo developper documentation. We also have a documentation dedicated to users.

• Coding guidelines
• Security and coding
• Branches and releases
• Git workflow and best pratices
• Database upgrade
• Technical changes in Piwigo version 16 / 15 / 14 / 13 / 12 / 11 / 2.10 / 2.8 / 2.7 / 2.6 / 2.5 / 2.4 / 2.3 / 2.2
• Send emails with Piwigo
• Multiple-Site-(Multisite)
• Logging

• Plugin developpement
• Plugin Tutorial: Hello world!
• Plugin Tutorial: Several tricks from the Copyrights plugin
• Use a source code manager for your extension
• How to create a theme
• Migrate a plugin from Piwigo.org SVN to Github
• Compilation of personal plugins
• Publishing an extension

• Piwigo Web API introduction
• pwg.images.addSimple
• pwg.images.add and pwg.images.addChunk
• pwg.images.upload