Do this instead and your error should go away:
$logo_arrray = explode('/', $header['css_logo_url']);
$bg_img_array = explode('/', $header['css_bg_img_url']);
$excl_imgs[] = end($logo_arrray);
$excl_imgs[] = end($bg_img_array);
You should always try and fix any Strict Standards warning and not just hide them.