Manual:$wgUploadDirectory

Details
The file system path of the folder where uploaded files will be stored.

This location needs to be writable by the web server. See Upload#Check directory security.

By default, uploaded files are actually stored in subfolders of $wgUploadDirectory. See Special:MyLanguage/Manual:Configuring file uploads for information, and how to disable this behavior.

There are other variables which are defined relative to this one. If you redefine this in your  then all dependent variables including   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 !