Mosaico not displaying images in editor
If you're getting a 403 error on the image, that's not a Resource URL error but a file permission error.

Right-click the line with the image that isn't loading and select &quot;Open in New Tab&quot;.  Reloading this new tab will make it quicker to test changes.
Check the following (report back here if necessary):
The user that your web server runs under;
The user PHP runs as (if it's different);
The owner of the file that's not loading;
The file permissions of the file that's not loading.
Also check for any .htaccess files in the image folder or any parent folders, and your web server logs.

Between these, you should find your problem.  Be mindful that there's both the immediate problem to fix, and also the root cause that led to this situation.
The file owner is going to be your PHP user, but the web server user is the one accessing it.  So either a) they're different users (which is OK, but makes the other variables above relevant) or b) there's a .htaccess file in place.