var current = 1;

function change() {
document.images.show.src = 'gallery/htk/'+current+'.jpg';
}

function next() {
if (current<6) {
current++;
change();
   }
else first();
}
function previous() {
if (current-1 > 0) {
current--;
change();
   }
else last();
}
function first() {
current = 1;
change();
}
function last() {
current = 6;
change();
}