This is a common mistake. When defining a FileField or an ImageField, you need to specifywhere the files are stored. This is done by specifying a relative path in the upload_to argument. Django will then store your files in a subdirectory as named, under the MEDIA_ROOT directory. But, don't put a leading slash in the relative path. Otherwise it will try to store the file at the system root directory.
MEDIA_ROOT = '/home/myname/files/ upload_to='pictures' file: abc.jpg results in: /home/myname/files/pictures/abc.jpg
MEDIA_ROOT = '/home/myname/files/ upload_to='/pictures' file: abc.jpg results in: /pictures/abc.jpg
Also a side tip: You can add strftime style arguments to the upload_to argument, storing the files in dated sub directory. e.g.
MEDIA_ROOT = '/home/myname/files/ upload_to='/pictures/%Y/%b/%d' file: abc.jpg results in: /pictures/2008/May/09/abc.jpg