The directory sites/default/files is not writable on Mac OS X
The files directory in your site must be writable by the webserver. In the finder, do a Get Info on the files directory and assign read &amp; write permissions to staff. (Make sure that this applies to any directories inside files as well.) Alternatively, in the terminal you could "chmod -R 775 my-site-path/sites/default/files" to take care of files and any contained subdirectories.