The alias of EDD Product image URL in Mautic is edd_product_image_url

This is how you can use edd_product_image_url custom field placeholder while composing Mautic Email.

<!-- // Lets assume you are writing a Mautic email -->
Hello {contactfield=firstname},

Thank you for purchasing {contactfield=edd_product_name}.
<img src="{contactfield=edd_product_image_url}" alt="featured-image">

Regards,
Your name
<!-- // Lets assume this is end of your Mautic email body -->

Hence, edd_product_image_url outputs the URL of your EDD downloads featured image.

Note: Make sure you have set featured image for the EDD Download’s product/service.