I did notice their graphic is too wide. You could resize via html or you could edit & rehost the graphic or, a simpler block would be to create a javascript/html block and insert the following:
<form target="weather" action=http://www.wunderground.com/cgi-bin/findweather/getForecast ><font color=#000000><p align="center"><font size="1">Find your Weather<br>Enter City, State, Zip, or Country</font></font><br><font color=#000000><input type=text name=query size=10><input type=submit value="Forecast"></font></form>