What browser are you in? The image is getting sized down so this is probably a side-effect of the way your browser resizes the images. I could probably fix it by making a separate image for that specific size.
I never changed my window size or zoomed in/out when I experienced the stars. I also can't reproduce them with what TBest wrote.
Next time I see this happening I try to click on the image to check whether this really is a separate image file or just a rendering issue. Furthermore, I will try how it will behave when I zoom in and what happens when I resize the window.