have you also tried a different banner design?

this is what appears in my signature box:

<span style="display: block !important; width: 320px; text-align: center; font-family: sans-serif; font-size: 12px;"><a href="http://www.wunderground.com/cgi-bin/findweather/getForecast?query=zmw:00000.18.10416&bannertypeclick=wu_clean2day" title="Unna, Deutschland Weather Forecast" target="_blank"><img src="http://weathersticker.wunderground.com/weathersticker/cgi-bin/banner/ban/wxBanner?bannertype=wu_clean2day_metric_cond&airportcode=EDLW&ForcedCity=Unna&ForcedState=Deutschland&wmo=10416&language=DL" alt="Find more about Weather in Unna, DL" width="300" /></a><br><a href="http://www.wunderground.com/cgi-bin/findweather/getForecast?query=zmw:00000.18.10416&bannertypeclick=wu_clean2day" title="Get latest Weather Forecast updates" style="font-family: sans-serif; font-size: 12px" target="_blank">Click for weather forecast</a></span>