div .collapsiblock a{
  background: url("https://www.sostp.cz/sites/all/themes/sostp/images/arrow_collapse.svg") no-repeat right;
  display: block;
}

div .collapsiblockCollapsed a{
  background: url("https://www.sostp.cz/sites/all/themes/sostp/images/arrow_expand1.svg") no-repeat right;
}