when you click into mail body you should see INSERT tab on the ribbon and there picture icon.
but limitation is that the picture has to be store in SP so mail SP has to be reachable to addressee to see the picture
Hey Robin Johnson,
if you want to have your logo in every Nintex mail in the whole SharePoint farm by default, you could also add the picture in the "Notification header" or "Notification Footer" section.
These sections can be found in the "Message templates" of Nintex Workflow on farm level and site collection level.
mail logo picture