firstImage = 1;
currentImage = 1;

function nextPicture() 
{
    // Hide current picture
    object = document.getElementById('slide' + currentImage);
    object.style.display = 'none';

    // Show next picture, if last, loop back to front
    if (currentImage == lastImage) { currentImage = 1; }
    else { currentImage++ }
    object = document.getElementById('slide' + currentImage);
    object.style.display = 'block';

    setTimeout(nextPicture, 5000);
}

function previousPicture() {
    // Hide current picture
    object = document.getElementById('slide' + currentImage);
    object.style.display = 'none';

    if (currentImage == firstImage) { currentImage = lastImage; }
    else { currentImage--; }
    object = document.getElementById('slide' + currentImage);
    object.style.display = 'block';
}