itabirit
I just installed a brand new instance of v1.18.1, left everything default, updated both User settings you did (Require login to create Tickets and set User Registration set to Private), registered a User, logged in as the User, and was able to upload an image and create the Ticket with no issues. This means something weird is going on with your install/setup.
As I recommended, you should setup a completely new and separate install of osTicket only in English and retest.
If you use a proxy you must forward the User's real IP via the X-Forwarded-For
header and then edit your ost-config.php file to include the Proxy addresses to the TRUSTED_PROXIES
option.
Cheers.