Where does one store a image for a letterhead in lightning?
THanks
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.
Just to add some further detail relating to the existing answer from @TusharSharma.
In order to see the Documents tab, you will need to switch to Salesforce Classic. To do this, click on your avatar in the top right and then click on the "Switch to Salesforce Classic" link. If you don't see the Documents tab at the top, click on the +
symbol at the far right of the tab headers and then make sure "All Tabs" is shown in the View dropdown - Documents should then appear in the list.