Resizing loses sharpness and quality, as does twisting -- my banner, the one I just talked about in my LJ? I tilted it yet really had to compensate for the loss of clarity.

I know some folks work on 200x200 canvas and then re-size, but in my opinion, their icons tend to be over-sharpened, no matter how good they are otherwise. The only way to retain the sharpness of the image is working on 100x100 px from the beginning, zooming in when necessary.