Hi Alessandro!
I checked right now and I found this …
Change the extension to this point into WU-forecast.php by removing the right side .jpg and entering .gif 
NWSiconlist = array(
// WU Icon name => NWS icon name // WU meaning
‘chanceflurries.gif’ => array(‘sn.gif’,‘Chance flurries’), // Chance of Flurries
‘chancerain.gif’ => array(‘hi_shwrs.jpg’,‘Chance rain’), // Chance of Rain
‘chancesleet.gif’ => array(‘ip.jpg’,‘Chance sleet’), // Chance of Sleet
‘chancesnow.gif’ => array(‘sn.jpg’,‘Chance snow’), // Chance of Snow
‘chancetstorms.gif’ => array(‘hi_tsra.jpg’,‘Chance thunderstorms’), // Chance of Thunderstorms
‘clear.gif’ => array(‘skc.jpg’,‘Clear’), // Clear
‘cloudy.gif’ => array(‘ovc.jpg’,‘Cloudy’), // Cloudy
‘flurries.gif’ => array(‘sn.jpg’,‘Flurries’), // Flurries
‘fog.gif’ => array(‘fg.jpg’,‘Fog’), // Fog
‘hazy.gif’ => array(‘fg.jpg’,‘Hazy’), // Hazy
‘mostlycloudy.gif’ => array(‘bkn.jpg’,‘Mostly cloudy’), // Mostly Cloudy
‘mostlysunny.gif’ => array(‘sct.jpg’,‘Partly cloudy’), // Mostly Sunny
‘partlycloudy.gif’ => array(‘sct.gif’,‘Partly cloudy’), // Partly Cloudy
‘partlysunny.gif’ => array(‘bkn.jpg’,‘Mostly sunny’), // Partly Sunny
‘rain.gif’ => array(‘ra.jpg’,‘Rain ‘), // Rain
‘sleet.gif’ => array(‘ip.jpg’,‘Sleet’), // Sleet
‘sleat.gif’ => array(‘ip.jpg’,‘Sleet’), // Sleet (other spelling)
‘snow.gif’ => array(‘sn.jpg’,‘Snow’), // Snow
‘sunny.gif’ => array(‘skc.jpg’,‘Sunny’), // Sunny
‘tstorms.gif’ => array(‘tsra.jpg’,‘Thunderstorms’), // Thunderstorms
‘unknown.gif’ => array(‘na.jpg’,’’), // Unknown
‘nt_chanceflurries.gif’ => array(‘nsn.jpg’,‘Chance flurries’), // Chance of Flurries
‘nt_chancerain.gif’ => array(‘hi_nshwrs.jpg’,‘Chance rain’), // Chance of Rain
‘nt_chancesleet.gif’ => array(‘ip.jpg’,‘Chance sleet’), // Chance of Sleet
‘nt_chancesnow.gif’ => array(‘nsn.jpg’,‘Chance snow’), // Chance of Snow
‘nt_chancetstorms.gif’ => array(‘hi_ntsra.jpg’,‘Chance thunderstorms’), // Chance of Thunderstorms
‘nt_clear.gif’ => array(‘nskc.jpg’,‘Clear’), // Clear
‘nt_cloudy.gif’ => array(‘novc.jpg’,‘Cloudy’), // Cloudy
‘nt_flurries.gif’ => array(‘nsn.jpg’,‘Flurries’), // Flurries
‘nt_fog.gif’ => array(‘nfg.jpg’,‘Fog’), // Fog
‘nt_hazy.gif’ => array(‘nfg.jpg’,‘Hazy’), // Hazy
‘nt_mostlycloudy.gif’ => array(‘nbkn.jpg’,‘Mostly cloudy’), // Mostly Cloudy
‘nt_mostlysunny.gif’ => array(‘nsct.jpg’,‘Partly cloudy’), // Mostly Sunny
‘nt_partlycloudy.gif’ => array(‘nsct.jpg’,‘Partly cloudy’), // Partly Cloudy
‘nt_partlysunny.gif’ => array(‘nbkn.jpg’,‘Mostly cloudy’), // Partly Sunny
‘nt_rain.gif’ => array(‘nra.jpg’,‘Rain ‘), // Rain
‘nt_sleet.gif’ => array(‘ip.jpg’,‘Sleet’), // Sleet
‘nt_sleat.gif’ => array(‘ip.jpg’,‘Sleet’), // sleet (other spelling)
‘nt_snow.gif’ => array(‘nsn.jpg’,‘Snow’), // Snow
‘nt_sunny.gif’ => array(‘nskc.jpg’,‘Sunny’), // Sunny
‘nt_tstorms.gif’ => array(‘ntsra.jpg’,‘Thunderstorms’), // Thunderstorms
‘nt_unknown.gif’ => array(‘na.jpg’,’’), // Unknown
‘.gif’ => array(‘na.jpg’,‘’), // Unknown
) ;
ok?