(Origiinal Link)
To debug image loading go to plugin settings > advanced settings. There enable the MPDF Debug mode:

https://www.welaunch.io/en/wp-content/uploads/sites/8/2017/09/MPDF-Debug-Mode-247x58.png 247w,https://www.welaunch.io/en/wp-content/uploads/sites/8/2017/09/MPDF-Debug-Mode-510x120.png 510w,https://www.welaunch.io/en/wp-content/uploads/sites/8/2017/09/MPDF-Debug-Mode-300x71.webp 300w,https://www.welaunch.io/en/wp-content/uploads/sites/8/2017/09/MPDF-Debug-Mode-768x181.webp 768w" alt="MPDF debug mode" width="861" height="203" class="size-full wp-image-5507 lazy-load-active" style="box-sizing: border-box; border-style: none; display: inline-block; height: auto; max-width: 100%; vertical-align: middle; opacity: 1; transition: opacity 1s ease 0s;" decoding="async" data-src="https://www.welaunch.io/en/wp-content/uploads/sites/8/2017/09/MPDF-Debug-Mode.webp" data-srcset="https://www.welaunch.io/en/wp-content/uploads/sites/8/2017/09/MPDF-Debug-Mode.webp 861w,https://www.welaunch.io/en/wp-content/uploads/sites/8/2017/09/MPDF-Debug-Mode-247x58.png 247w,https://www.welaunch.io/en/wp-content/uploads/sites/8/2017/09/MPDF-Debug-Mode-510x120.png 510w,https://www.welaunch.io/en/wp-content/uploads/sites/8/2017/09/MPDF-Debug-Mode-300x71.webp 300w,https://www.welaunch.io/en/wp-content/uploads/sites/8/2017/09/MPDF-Debug-Mode-768x181.webp 768w" />

Look what the image errors show. Maybe you find the issue yourself.

If images still do not display you can solve the issue by the following ways:

Quick and Dirty:

Add the following to the mpdf.php file on line 12954 -> function: file_get_contents_by_curl()

curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);

Source: https://stackoverflow.com/questions/5657382/curl-php-restful-service-always-returning-false

Better:

The better solution is to manually add only the certificate(s) or CA(s) you want to accept

;;;;;;;;;;;;;;;;;;;;
; php.ini Options  ;
;;;;;;;;;;;;;;;;;;;;

curl.cainfo = "C:\xampp\php\extras\ssl\cacert.pem"

Source: https://stackoverflow.com/questions/24611640/curl-60-ssl-certificate-unable-to-get-local-issuer-certificate

On Nginx?

Some nginx server disallow some agents in the CURL header. Therefore you will need to change the header. Go to plugin-folder/includes/mpdf/mpdf.php > Line: 12965.

Then change the CURLOPT_USERAGENT value to ‘User-Agent: curl/7.39.0’

Example:

curl_setopt($ch, CURLOPT_USERAGENT, 'User-Agent: curl/7.39.0');