That sounded like a great "why didn't I think of that" idea, but it didn't work exactly like that. If I right-clicked on the default logo and clicked properties, it says it was 150x50. So I set my logo to 150x50 and uploaded it.
Guess what? They shrunk it down and it looked terrible. So next I figured out if I right-click on >my< newly-uploaded terrible-looking logo, the properties show a size of 71x50. So I resized the original to 71x50 and uploaded again. That worked - it looks the way it should.
So the lesson here is that they allow 150x50 for their OWN logo, but only 71x50 for a paying customer's custom logo. Nice, huh?
Anyway, problem solved. Thanks for your help.