I have the same issue. See log with error. Nick, can you help?
[Tue Jan 28 15:38:59 2014] [error] [client 172.26.147.137] PHP Fatal error: Uncaught exception ‘Exception’ with message ‘Required GD library is missing’ in /var/www/html/wp-content/plugins/salesforce-wordpress-to-lead/lib/captcha/captcha.php:92\nStack trace:\n#0 /var/www/html/wp-content/plugins/salesforce-wordpress-to-lead/salesforce.php(492): captcha()\n#1 /var/www/html/wp-content/plugins/salesforce-wordpress-to-lead/salesforce.php(838): salesforce_form(Array, false, NULL, 2)\n#2 [internal function]: salesforce_form_shortcode(Array, ”, ‘salesforce’)\n#3 /var/www/html/wp-includes/shortcodes.php(236): call_user_func(‘salesforce_form…’, Array, ”, ‘salesforce’)\n#4 [internal function]: do_shortcode_tag(Array)\n#5 /var/www/html/wp-includes/shortcodes.php(151): preg_replace_callback(‘/\\[(\\[?)(embed|…’, ‘do_shortcode_ta…’, ‘<p>We c…’)\n#6 [internal function]: do_shortcode(‘<p>We c…’)\n#7 /var/www/html/wp-includes/plugin.php(173): call_user_func_array(‘do_shortcode’, Array)\n#8 /var/www/html/wp-content/plugins/advanced-custom-fields/core/fields/wysiwyg.php(277): apply_filt in /var/www/html/wp-content/plugins/salesforce-wordpress-to-lead/lib/captcha/captcha.php on line 92