I can confirm that issue on a vmware based Ubuntu 18.10 SMP 64 Bit / PHP 7.2.10-0ubuntu0.18.04.1 (cli) (built: Sep 13 2018 13:45:02) ( NTS ). Alle the mentioned extensions are installed (mbstring, curl, ctype extension, gdlib)
I do a fresh
git clone https://github.com/getkirby/starterkit.git kirby30,
start the built in PHP server (
php -S localhost:8000) an will see not a single image in frontend - only black boxes - and no image in panel.
PHP outputs nothing helpully, only 404 like
127.0.0.1:58004 : /media/pages/photography/animals/3779077927-1547930887/peter-fox-800x1000-q90.jpg - No such file or directory
In panel I get a
JSON.parse: unexpected character at line 1 column 1 of the JSON data when clicking down the image placeholders. Maybe this helps.