Hey! Designer at Ghost here. @Cathy_Sarisky is right – we’ve currently got a private beta running that gives you more control over who should see the content inside your HTML card.
That said, we’re working on extending this functionality to a new call-to-action card which lets you do exactly what you asked: creating an ad block with an “advertisement” label, an image, background color, etc. That way, you won’t have to create everything yourself in HTML. I can’t tell you exactly when it will be released, but it’s coming soon!