As per Salesforce docs:

    

> You must upload the logo image to the Documents tab before you can add
> it to the letterhead. Make sure the document is marked as an
> Externally Available Image so it is visible to non-Salesforce users
> when it is received in an email.

So Document is the answer of your question.

[Create Letterheads for Email Templates][1]

Update: If document tab is not available that doesn't mean you cannot use them in Lightning. From docs:

> The Documents tab isn’t available in Lightning Experience, but your
> document files aren’t going away. In fact, some features continue to
> rely on files in your Documents tab, like email templates.

[Moving Documents to Salesforce Files][2]


  [1]: https://help.salesforce.com/articleView?id=creating_letterheads.htm&type=5
  [2]: https://help.salesforce.com/articleView?id=docs_documents_to_files.htm&type=5