Update add_options_page call with capability

Numeric capabilities are deprecated since WP 2.0. Replaced with string
capability.
Fix HTTPS check

Some servers (XAMPP) do not define $_SERVER['HTTPS'] if connection is
unencrypted. Added a check to prevent PHP warnings.
Fix undefined constant 'on' warning

Changed "on" constant to a literal.
Fix undefined variable post_image

Ensured post_image is set when there are no images in the feed.