There is no convention and no rule on this. The fact that it's common to have the type below the graphic is partly that it's currently fashionable and partly a design decision based on what the client or the design feels should have prominence.
There is only one case where going with what's conventional is actually required in a design, and that's when you want to appear conventional. That's not intended as a frivolous statement, for all it seems simplistic. There are plenty of occasions when this is an important consideration in a design. Similarly, there are occasions when you must to do the unconventional: when the client needs to appear unconventional, you'd be failing in your job if you used Time Roman or Helvetica for their wordmark.
In crafting your mark, definitely consider both the typography and the graphic as a single entity. They will work together how they work, and if the typography belongs on top then that's where it should be. But do base your decision on a good reason. A good reason is one that can be articulated and is not merely an opinion or feeling, such as "the graphic gives a solid base to the name and makes it stable and balanced."
"It's not usually done that way" or "it feels right" are not good reasons.