Sencha comes with at least two animated "spinner" gifs: loading.gif and large-loading.gif. The first (loading.gif) does not have a transparent background and the second only looks good on a light-colored canvas (e.g., a white or grey background).
I needed a transparent spinner animation that worked on a dark backgrounds, so I modified loading.gif and thought I'd share the results here in case it's useful to anyone else.
Again, the edges will look slightly rough if you view it on a white background, but with dark backgrouds it looks great. (It's also about half the file size of the other spinners.)