Thanks for your help, tophand. I removed and reinstalled IH2, but that didn't seem to help (even after I remembered to redo the settings in the admin ). I ended up uploading includes/modules/pages/popup_image/header_php.php and includes/modules/pages/popup_image_additional/header_php.php from the IH2 download (overwriting the files from 1.3.8) and it's behaving normally now.