Problem assigning a tween

I have a gallery where passing over the image a rectangle drops down showing the caption for the image.

The informations are read from an XML file and some of my images don't have a caption, for those emails I want to avoid the tween.

The XML file has this structure:

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<images>
     <pic>
        <image>gallery/certificato.jpg</image>
        <thumbnail>gallery/certificato_s.jpg</thumbnail>
        <caption>none</caption>
        <desc>none</desc>
    </pic>

</images>

The code where the tween are associated to the images is this

function initGallery()
{
    function loadXML(loaded)
    {
        if (loaded)
        {
            xmlNode = this.firstChild;
            total = xmlNode.childNodes.length;
            for (i = 0; i < total; i++)
            {
                _root.small_image = xmlNode.childNodes.childNodes[1].firstChild.nodeValue;
                _root.big_image = xmlNode.childNodes.childNodes[0].firstChild.nodeValue;
                _root.description = xmlNode.childNodes.childNodes[2].firstChild.nodeValue
                _root.testo = xmlNode.childNodes.childNodes[3].firstChild.nodeValue;
                if (i == 0)
                {
                    /* added the last parameter accordingly to the modified loadGImage */
                    _root.loadGImage(_root.description, _root.big_image, _root.testo, i);
                } // end if

                if(_root.testo != "none") { //shadowbox tween in
                    _root.bigImage.onRollOver = function() {
                        var shadowboxIn:Tween = new Tween (shadowbox_mc, "_y", Strong.easeOut, -49, 0, .5, true);
                    }
                }
               
                if(_root.testo != "none") { //shadowbox tween out
                    _root.bigImage.onRollOut = function() {
                        var shadowboxOut:Tween = new Tween (shadowbox_mc, "_y", Strong.easeOut, shadowbox_mc._y, -49, .5, true);
                    }
                }
               
                ++_root.total_images;
            } // end of for
            createSmall();
        }
        else
        {
            content = "file not loaded!";
        } // end else if
    } // End of the function

    xmlData = new XML();
    xmlData.ignoreWhite = true;
    xmlData.onLoad = loadXML;
    if (_root.xml_file == undefined)
    {
        _root.xml_file = "images.xml";
    } // end if
    xmlData.load(xml_file);
} // End of the function

I try to avoid the tween checking whether the caption value is "none" or not, but that doesn't work, the tween is always applied.

How can I do?