What is currently required is that the logo is 50x50 and under 50kb. Even if you allowed .PNG, the logos would continue to be under 50kb. As displayed below:
Both of them are under 50kb
.PNG:
JPG:
And surprisingly the .PNG is substantially smaller, at only 4.06kb. And remaining quite a bit sharper. While the .JPG is over 15kb, and is quite blurry.
-Unknown