The current system is really hard to use efficiently in my opinion. I like the idea of automatically aligning images, but it gets in the way whenever I want to make something that isn't center-aligned, and I don't think there's any way to turn it off.
I think there should just be a button that says something like "center art" next to the clear one, so anyone who wants their sprite to be centered can have it that way, while if I want my image to be off-center, I can have that too.
Edit: Scratch also has a small crosshair to indicate the center of the art editor. I think this is a really important thing that Snap's art editor is also missing.