path for watermark image in linux

[eluser]PHPraja[/eluser]
I m using centos (linux) for my project. For creation of watermark images or thumbnails i m specifying the path in the below manner. In windows its working well. but in linux this is creating problem. No watermark image is genetated.

Code:

$config['source_image'] = './photo_uploads/'.$f['raw_name'].$f['file_ext'];
$config['wm_overlay_path'] = './photo_uploads/mego_footer_logo.png';

should i use a absolute path? should i use base_url() or site_url() ? I tried but no use. Can any one help me how to specify the path, or where i went wrong.

[eluser]PHPraja[/eluser]
ok guys, sorry, the path might be not the problem, GD installation in linux is the problem