var iIndex = 1;

function fncOtraIMG(){ 
    document.getElementById("img" + iIndex).style.display = "none";
    if (iIndex<5){
        iIndex = iIndex + 1;        
    }else{
        iIndex = 1;
    }
    document.getElementById("img" + iIndex).style.display = "";
    document.getElementById("input_hdnIndex").value=iIndex;    
}