var imgRe = /^.+\.(jpg|jpeg|gif|png)$/i;

function previewImage(pathField, previewName)
{       
    var path = pathField.value;
    if (path.search(imgRe) != -1)
    {   
        document[previewName].src = 'file://'+path;
    }       
    else    
    {   
        alert("JPG, PNG, and GIFs only!");
    }   
}