Thank you for this information. Unfortunately, this does not come inline with our current core development road map.

We will consider this in the future.

Success template is shared with few different submit processes.  It will not be right to add it to that template.

You can add javascript to success controller. It will be loaded right after order is placed.

/storefront/controller/pages/checkout/success.php

Add:
$this->document->addScript(RDIR_TEMPLATE . 'javascript/[YOUR FILE]');
somewhere after  line:
$this->extensions->hk_InitData($this,__FUNCTION__);

Can we see URL to this page? How big is your original image?

Two  common reasons and solutions for this error "MySQL server has gone away Error No: 2006"

1. Mysql timed out and closed the connection.
Solution: Check wait_timeout setting in /etc/mysql/my.cnf configuration file is large enough. Try to set wait_timeout with larger value. After change restart mysql with sudo /etc/init.d/mysql restart.

2.  Server drop large packet. If mysql gets a packet that is too large or incorrect, it assumes that something has gone wrong with the client and closes the connection. You can increase the maximal packet size limit by increasing the value of max_allowed_packet in /etc/mysql/my.cnf.
After change restart mysql with sudo /etc/init.d/mysql restart.

If you do not have root privileges, check with your hosting company

It sounds like your image directory is not set to work with SSL correctly. Can you post your site URL?

This could be a small bug in html in case of empty category. But does it make sense to have empty category?

cron jobs command of AbanteCart

    What is that?
Cron job is ran on the UNIX/Linux OS.

I assume you need to automate something for AbanteCart. What is the process you need to automate?

You can have PHP code in the template files (.tpl), if this is what you mean.

Template files have scope of view class and you can have anything available in view accessible in the template.