Manual:$wgGalleryOptions

Details
The default settings for the   tag, and galleries in categories.

See the "default value" below for the possible keys, and a description of each setting.

Some of these settings correspond, in a confusing way, to attributes that can be specified for the   tag itself.

See option>Special:MyLanguage/Help:Images#Optional gallery attributes|Help:Images#Optional gallery attributes for information on each of the attributes.