bernaldezglaiza,
Sorry to hear you’re having trouble here, but hopefully I can help. The Community User account shouldn’t have permissions to upload images to the back-end of the site. The Community add-on creates a new page for users to submit events using the front-end of the site.
After logging in, your users should be taken to your Event Submission page where they can submit events and upload images. Try logging in as the Community User and going to your Event Submission page to see if you’re still having the same issue.
Thanks! ๐
-Casey-