test.html

<script type="text/javascript">
    function change_gif(selectObject) {
　　　　var img_obj = document.getElementById("mapimg");
        var opt_value = selectObject.value;
        
        switch (opt_value){
            case "precip":
                img_obj.innerHTML='<img src="https://raw.githubusercontent.com/Novarizark/wrf-sdpwfe/master/fig/precip.gif?'+ (new Date().getTime())+'"/>';
                break;
            case "snow_cover":
                img_obj.innerHTML='<img src="https://raw.githubusercontent.com/Novarizark/wrf-sdpwfe/master/fig/snow-cover.gif?'+ (new Date().getTime())+'"/>';
                break;
        }
　　}
        
</script>

<select style="margin-bottom: 15px" id="select01" onchange="change_gif(this)">
  <option value="precip">Precipitation</option>
  <option value="snow_cover">Snow Cover</option>
<!--  <option value="t2m">2m Temp & Relative Humidity</option>
  <option value="wind">10m Wind</option> -->
</select>
<div id="mapimg"></div>