@hiccup, the way MB works with bitmap skins for the progress bar is MB uses the left side of the image provided as the left side of the progress bar, the middle section of the provided image as the fill colour, and the right side as the right side of the progress bar. Thats fine with your skin.
When playback starts MB uses the "Filler" image. In your case you have set it to a different size to the first image. It needs to be the same size and has the same behavior as the first image, except it only displays when playback has started and only applies to the left side of the progress bar.
I suggest you just copy ProgressBar.png to ProgressBarFill.png and put the green fill colour in the middle. Or if you want the arrows wiped out on playback then fill the entire image with green.
I have made one improvement for the next update for handling the progress bar width being smaller than the left/right side of the progress bar image. So until that update is made available it still wont work optimally for you