|Shared uploads: $wgUseSharedUploads|
|Whether to look in a shared repository when a file is not found locally|
|Introduced in version:||1.3.8|
|Removed in version:||still in use|
|Other settings: Alphabetical | By function|
If you operate multiple wikis, you can define a shared upload path using $wgSharedUploadPath. If $wgUseSharedUploads is set, the wiki will look in the shared repository if no file of the given name is found in the local repository (for [[Image:..]], [[Media:..]] links).
Thumbnails will also be looked for and generated in this directory.
Uploads to the wiki will NOT be put there - they will be put into $wgUploadDirectory.
For resizing images in mediawiki they need to have a row in image table of database containing dimensions and other information of image so you can't resize images that haven't been uploaded by MediaWiki software and for importing them to database without using MediaWiki interface, you'll need to use some maintenance scripts (maintenance/importimage.php).