We don't have the full specs written out anywhere. That would be a good idea. I can answer your questions, though.
Categories are not automatically created.
Images aren't included. They will be referenced in the body of the profiles if they're referenced in the GEDCOM.
You are right that the WikiTree IDs can be ignored.
"Everything else" means everything else. :-) Our goal with the import is not to lose any information from the GEDCOM. If there's not a field for something in the database, it's left in the text of the profiles.