Manual talk:$wgHashedUploadDirectory

changing an existing configuration
You will receive following error (MW 1.9.2)

 Warning: imagecreatefrompng(E:\Programme\xampp\htdocs\w/images/1_GrafikTest_01_1024.png) [function.imagecreatefrompng]: failed to open stream: No such file or directory in E:\Programme\xampp\htdocs\w\includes\Image.php on line 1266

Warning: imagecopyresampled: supplied argument is not a valid Image resource in E:\Programme\xampp\htdocs\w\includes\Image.php on line 1274

Fatal error: imagepng [function.imagepng]: gd-png: fatal libpng error: Invalid number of colors in palette in E:\Programme\xampp\htdocs\w\includes\Image.php on line 1275