Manual:$wgSharedUploadDirectory

Details
Path on the file system where shared uploads can be found. 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, [[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.

Thumbnails of shared images
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 maintainance scripts.(maintanance/importimage.php) If those images have been uploaded in another mediawiki, you can set $wgSharedUploadDBname to solve the problem.