Getting templates images on Mosaico
It turns out this is actually to do with the base page url - it should not have a trailing slash. @massetto knows this I think, because he created the GitHub issue where this is explained so this answer is really for anyone else looking at this later.
From Matt Wire of Veda:

Check your CiviCRM paths/resource URLs and make sure that you are not overriding them if you don't need to.
In CiviCRM make sure that &quot;Wordpress Base Page&quot; is setup as &quot;civicrm&quot; with no &quot;/&quot; - see civicrm/civicrm-core#18332 so this will be fixed in 5.29.
In Wordpress make sure you have a CiviCRM base page (should be at https://yoursite.com/civicrm).
Important: Go to Settings -&gt; Permalinks and click &quot;Save Changes&quot;.
Now Mosaico images should work and navigating to https://yoursite.com/civicrm/mosaico/img should give a 200 OK and a blank page.

v5.29 is available now, so sounds like upgrading to that should resolve the problem.