Warning: imagejpeg() [function.imagejpeg]: Unable to open \'/home/SITENAME/public_html/files/cache/052f225905c1618003df0c5088aec7a9.jpg\' for writing: Permission denied
For future reference the problem was the PHP handler. It has been changed to CGI mode (as opposed to DSO) and they turned suEXEC ‘off’ - might be useful for someone down the line.