It worked ok but I don't like to see a great swag of code repeated so I rewrote it as follows:

a more succinct counter_clicked function
function counter_clicked(el) {
  var id=el.id;
  var n=id.substring(1);
  if (n<10) {
    var row=n;
    for (var col=0;col<10;col++) fill(row,col);
  } else {
    var col=n-10;
    for (var row=0;row<10;row++) fill(row,col);
  }
}
function fill(row,col) {
  var id=rc2id(row,col);
  var el=document.getElementById(id);
  var v=el.getAttribute('v');
  if (v==0) {
    var a=get_a(row,col);
    if (a==7) {
      el.src='images/right/7.png';
    } else {
      el.src='images/wrong/7.png';
    }
  }
}

You'll have noticed the new JavaScript word, substring.
As usual, we start counting at 0 so the 1 means to start at the second character.