The 256x256 ones are a bit pixilated. Looks like you created them simply by creating a 256x256 icon from the 48x48 icon. It's usually better to make a 256x256 icon and then make the others from that one to keep the edges nice and smooth. Visuals are pretty good though.

When I look at these, they are quite nice. I see the potential of arranging the dark spots on the rings to give a "dented" appearance to them. A dented brass theme would be cool. If you did so, you would just have to take into account the way that the light would reflect off the dents- where they would be lighter or darker.