Internal Server Error 500 – blog not showing up

My blog has been down since Friday with the above error coming up instead. I can’t access the dashboard or any other part of my wordpress either. The error log shows the follow problem, does anybody know how to fix this? I don’t know a lot about how wordpress works

PHP Warning: imageline(): supplied argument is not a valid Image resource in /hermes/web06/b1938/pow.emmacrees/htdocs/blog/wp-content/themes/default/images/header-img.php on line 70
PHP Warning: imagecolorallocate(): supplied argument is not a valid Image resource in /hermes/web06/b1938/pow.emmacrees/htdocs/blog/wp-content/themes/default/images/header-img.php on line 65