There are problems in IE8 when you call .maphighlight() on an image that is not visible, either because its parent container or the image itself has display: none applied to it. Upon subsequently making the container visible, the entire image is not shown.
There is another bug I've found where the map highlight intermittently doesn't work in IE8 however I suspect it may be fixed when this one is resolved.
HI, I have the same problem.
hide image container after init maphighlight... (set filter/opacity=0 for container to prevent blink)... dirty code (=