There's an error in base full image.tpl (introduced in EZP-9160 / commit: 164c62c3).
Variables are being created with {def but their release is being attempted with {/let

IMHO, this issue is of very little importance, for a couple of reasons.
The template with the error is probably overridden in new designs.
The legacy templates are scheduled to be removed

Nevertheless, I'll attempt to submit a PR to fix this (as soon as my fork gets updated with upstream - worst internet connection ever )

Steps to reproduce:

run php bin/php/eztemplatecheck.php | grep "file invalid"
=> design/base/override/templates/full/image.tpl will be listed as invalid