function changePicture(c){
    akt = c;
    document.getElementById("ga_randpic").src = img[c];
    document.getElementById("img_text").innerHTML = txt[c];
}

function stepPicture(i){
    var last = img.length - 1;
    var next = akt + i;
    if(next < 0) {
        next = last;
    } else if (next > last) {
        var next = 0;
    }
    akt = next;
    document.getElementById("ga_randpic").src = img[next];
    document.getElementById("img_text").innerHTML = txt[next]; 
}