Rollover image unintentionally moving by one pixel

Hi. I have a small image that I use as a Back button. I've set the state and trigger to make it look like it's highlighted yellow when the mouse is hovered over it. What happens is this--the button is positioned on the screen, I roll over it, it turns yellow, and when I move my mouse off of it, the original button jumps up by about one pixel. It should be totally static. This has happened in another course as well with a marker.

I've included the file. Thanks for your help.