Manual:$wgCountCategorizedImagesAsUsed

Special pages: $wgCountCategorizedImagesAsUsed
On Special:UnusedFiles, consider images "used" if they are put into a category.
Introduced in version:1.4.1
Removed in version:Still in use
Allowed values:(boolean)
Default value:false

Details

On Special:UnusedFiles, consider images "used" if they are put into a category. Default (false) is not to count those as used.