which_image_loaded = 0;
NUMBER_OF_IMAGES = 6;

ImageNames = new Object();
ImageNames.length = NUMBER_OF_IMAGES - 1;

for (counter = 0; counter < NUMBER_OF_IMAGES; counter++){
    file_number = counter + 1;
    filename = ("images/alarmpro" + file_number + ".png");
    ImageNames[counter] = filename;
}

function changeImage(direction) {
    which_image_loaded += direction;
    if (which_image_loaded < 0)
        which_image_loaded = NUMBER_OF_IMAGES - 1;
    if (which_image_loaded == NUMBER_OF_IMAGES)
        which_image_loaded = 0;
    if (document.images)
        document.alarmpro.src = ImageNames[which_image_loaded];
}