ok, now i have to agree with you all. after i update firefox to 2.0.0.6, that problem occurs. in ie, there is also another broken image at search box. i do the same way as da clown do, look at the source and get this
<html><body><img alt="The image “http://www.rejetto.com/forum/Themes/default/images/filter.gif” cannot be displayed, because it contains errors." src="http://www.rejetto.com/forum/Themes/default/images/filter.gif"></body></html>
which means the message is only alt text, and show up because the real picture is not there. this should be the problem