Edit in JSFiddle

var panoramioLayer;
function initialize()
{
    var map = new google.maps.Map(document.getElementById("map"),
    {
        zoom: 3,
        center: new google.maps.LatLng(33.8869, 9.5375),
        mapTypeId: google.maps.MapTypeId.ROADMAP
    });

    panoramioLayer = new google.maps.panoramio.PanoramioLayer();
    panoramioLayer.setMap(map);
}

function getTaggedPhotos()
{
    var tagFilter = document.getElementById('tag').value;
    panoramioLayer.setTag(tagFilter);
}

<html>
<head>
<title>Google Maps API v3 Example: Panoramio Layer Tag search</title>
<script type="text/javascript" src="http://maps.google.com/maps/api/js?sensor=false&libraries=panoramio"></script>
<script type="text/javascript">
</script>
</head>
<body onload="initialize()">
<div style="height: 30px; width: 530px" align="center">
<input id="tag" type="textbox">
<input type="button" value="Search" onclick="getTaggedPhotos()">
</div>
<div id="map" style="height: 200px; width: 530px"></div>
</body>
</html>