Manual:$wgUploadDirectory

Details
The file system path of the folder where files will be uploaded. If one wants to have separate resources folder per wiki yet sharing same source code then it can be changed to below in their corresponding  file.

''Note: There are other variables which are defined relative to this one. If you redefine this in your LocalSettings.php then all dependent variables including wgUploadPath will need redefining also. If both these settings are not coherent the files will be uploaded to different folder than from where MediaWiki will try to retrieve it for viewing !''