There are a number of reasons for gift card images not showing in the email. To resolve this issue, kindly try the solutions below.

1. Please, note that for correct sending of images via emails, the module requires installation of the Imagick module on the server as well as dompdf library. Therefore, as the first step, make sure that Imagick module and dompdf library are installed. You may install the extensions via composer with the following commands:

composer require dompdf/dompdf
sudo apt-get install php-imagick

2. If the modules required are already installed, then, please, try to adjust the /etc/ImageMagick-6/policy.xml file by adding the following line: <policy domain="coder" rights="read|write" pattern="PDF" /> - to the end of the file. The final code should look as follows: 

.....
<policy domain="coder" rights="none" pattern="XPS" />
<policy domain="coder" rights="read|write" pattern="PDF" />
</policymap>