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
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.