Hi,
The image added by the plugin "attach serial" is generated dynamically.
You do the right thing by clicking on the button "email" in the order.
But you have introduced a PHP warning by modifying the email "order_notification". You have to fix this problem if you want to remove the notice.
Which version of HikaSerial are you using ?
The plugin AttachSerial will attach the image for every emails when the order status is right (depending on your HikaSerial configuration). If you didn't change the order status, you should see the attachment, as you can see the HikaSerial content in in the email itself.
If you do not see the attachment, you might not use the latest version. In HikaShop we introduce the email "order_notification" for the email button in a quite recent release, his support in HikaSerial as been introduced in the latest build (1.8.0).
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.