We use a header image on our notifications but I have the entire notification structured in a table that is 800px wide, so I just have the image set for width=100% within the top cell of the table.
there are no specifications since it’s not wholly controlled by Docebo...you’re using an email engine to send emails and different email software can behave differently and render differently…
As @sjennings78 says, this is what we do and I recommend as a best practice is to use tables in your emails. This will keep things pretty tidy and uniform regardless of the email software. we used one of the templates already developed by Docebo and enhanced it. The notification, “learner has enrolled in a course” has a template you can use...
When it comes to emails, always fall back to 1990’s web design standards to have control, table it up! :)