function next()
{
    var image_count = document.getElementById('image_count').value;
    var image_set   = document.getElementById('image_set').value;
    var max_sets = Math.ceil(image_count/6); 

    if (image_set < max_sets && max_sets != 1) {
        document.getElementById('set_' + image_set).style.display = 'none';
        image_set++;
        document.getElementById('set_' + image_set).style.display = 'block';
        document.getElementById('image_set').value = image_set;
    }
    //image_set.value = image_set++;    
}

function prev()
{
    var image_count = document.getElementById('image_count').value;
    var image_set   = document.getElementById('image_set').value;
    var max_sets = Math.round(image_count/6); 

    if (image_set > 1) {
        document.getElementById('set_' + image_set).style.display = 'none';
        image_set--;
        document.getElementById('set_' + image_set).style.display = 'block';
        document.getElementById('image_set').value = image_set;
    }
    //image_set.value = image_set++;
}