Try:
var check = setInterval(function() { Check(); }, 5000);
function Check() {
if(value != null) {
document.getElementById("li_tag").style.listStyleImage = "url('images/Correct.png')";
document.getElementById("li_tag").style.color = "#58BB27";
} else {
document.getElementById("li_tag").style.listStyleImage = "url('images/Error.png')";
document.getElementById("li_tag").style.color = "#B70000";
}
}