This doesn't apply to all cases, but in the example given... why are you using images at all? The button style could easily be created using CSS3 and allow you to sidestep all this inconvenience.
I believe rounded corners do not render nicely with CSS3 on Android's implementation of WebKit. At least on my device, if you create rounded corner buttons using CSS3, they end up looking jagged in the browser.
Android's WebKit implementation lacks anti-aliasing for border radius prior to 3.x.