more than a missing feature
So it's not be fixed ? because this bug its in Start10 and Start11.
Not sure if it'll ever be implemented or not. I never spent much time using Start10 because of how limited it is but I'm sure you can find a way to adjust the image proportions if you're using a custom image instead of one included with Start10 or the WindowBlinds skin, though with the latter you could also adjust the image used in the skin with SkinStudio if you're comfortable doing that.