I think what you're seeing is the middle image in blue sandwiched between two converted images.

I can see why this would happen.

For IE6 support we had the image as a gif as it had transparency yet needed to be given a dynamic size. IE6 didn't support PNGs without a fudge, and that fudge could not then do full CSS to style the images.

Additionally gif used to suffer from patents, meaning PHP could not read them. That patent is now expired, but some PHP versions (of GD library versions) still haven't had gif support re-enabled.

So, for future releases we will make it a gif.