function validate(pictureFieldId)
{
var extensions = new Array("jpg","jpeg","gif","png");

/*
// Alternative way to create the array

var extensions = new Array();

extensions[1] = "jpg";
extensions[0] = "jpeg";
//extensions[2] = "gif";
extensions[2] = "png";
extensions[3] = "";
*/

    var picturefieldValue=$F(pictureFieldId);

    if (picturefieldValue==null)
    {
        alert ('An error has occured: Javascript Libraries Conflict');
    }
    var image_file = picturefieldValue;
    var image_length = picturefieldValue.length;
    if (image_length<=0)
    {
        return true;
    }

var pos = image_file.lastIndexOf('.') + 1;

var ext = image_file.substring(pos, image_length);

var final_ext = ext.toLowerCase();

for (i = 0; i < extensions.length; i++)
{
    if(extensions[i] == final_ext)
    {
    return true;
    }
}

alert("You must upload an image file with one of the following extensions: "+ extensions.join(', ') +".");
return false;
}