Plugin generating PHP Warining in logfiles

Hey, thanks for this great plugin!
I encouter strange PHP Warnings which lead to timeouts (look at the timecode – its producing about 20 warnings per second!):

[Fri Dec 02 11:04:10.092328 2016] [fcgid:warn] [pid 9152] [client 66.249.76.0:54537] mod_fcgid: stderr: PHP Warning: Missing argument 3 for PIGEN::pigen_wp_get_attachment_image_attributes(), called in /*/wp-includes/plugin.php on line 235 and defined in /*/wp-content/plugins/pdf-image-generator/pdf-image-generator.php on line 607

[Fri Dec 02 11:04:10.099575 2016] [fcgid:warn] [pid 9152] [client 66.249.76.0:54537] mod_fcgid: stderr: PHP Warning: Missing argument 3 for PIGEN::pigen_wp_get_attachment_image_attributes(), called in /*/wp-includes/plugin.php on line 235 and defined in /*/wp-content/plugins/pdf-image-generator/pdf-image-generator.php on line 607

[Fri Dec 02 11:04:10.099599 2016] [fcgid:warn] [pid 9152] [client 66.249.76.0:54537] mod_fcgid: stderr: PHP Warning: Missing argument 3 for PIGEN::pigen_wp_get_attachment_image_attributes(), called in /*/wp-includes/plugin.php on line 235 and defined in /*/wp-content/plugins/pdf-image-generator/pdf-image-generator.php on line 607