function LoginButonOnclick() {
  var data= {
  UserName: $('#UserNameTextBox').val(),
  Password: $('#PasswordTextBox').val(),
  };
  if (data.UserName== null) {
    $("#showwarning1").html('<img src="~/Image/warning.png">');
  }
  if (data.Password== null) {
    $("#showwarning2").html('<img src="~/Image/warning.png" />');
  }
}

<input type="text" id="UsernameTextBox" name="UsernameTextBox"/>
<input type="text" id="PasswordTextBox" name="PasswordTextBox"/>
<input type="button" onclick="LoginButonOnclick()" value="Enter"/>

<div id="showwarning1"></div>
<div id="showwarning2"></div>

<img src="~/Image/warning.png" title="Password is empty!!"/>

function LoginButonOnclick() {
  var data= {
  UserName: $('#UserNameTextBox').val(),
  Password: $('#PasswordTextBox').val(),
  };
  if (data.UserName== null) {
    $("#showwarning1").html('<img src="~/Image/warning.png">');
  }
  if (data.Password== null) {
    $("#showwarning2").html('<img src="~/Image/warning.png" />');
  }
}
<input type="text" id="UsernameTextBox" name="UsernameTextBox"/>
    <input type="text" id="PasswordTextBox" name="PasswordTextBox"/>
    <input type="button" onclick="LoginButonOnclick()" onmouseover="LoginButonOnclick()" value="Enter"/>

    <div id="showwarning1"></div>
    <div id="showwarning2"></div>

$(function(){
 $("#showwarning1").on('mouseover', function(){
  //show error message to the user
  console.log("UserName Can Not Be Empty!");

 });

 $("#showwarning2").on('mouseover', function(){
  //show error message to the user
  console.log("Password Can Not Be Empty!");
 });
});