According the eZ/Publish/Core/FieldType/Image/Type.php:121 the alternative text is optinal.

Creating images without an alternative texts works fine using the inner repository services or the backend drag&drop image creation.
Creating the image through the default content edit requires the alternative text is filled.
The reasons for that is that the field is not marked as optional inside the form building: EzSystems\RepositoryForms\Form\Type\FieldType\ImageFieldType:40.

Steps to reproduce:
1. Create a new CT in "content" group.
2. Add an Image FieldType and mark it as "required".
3. Save the new CT.
4. Create a new content object using this CT.
5. Try to publish the content without specifying the alternative text.

Result:
Content could not be published, the alternative text becomes required.

Expected result:
The alternative text should remain optional.