How to send image to PHP file using Ajax?

Use JavaScript’s formData API and set contentType and processData to false

$("form[name="uploader"]").on("submit", function(ev) {
  ev.preventDefault(); // Prevent browser default submit.

  var formData = new FormData(this);
    
  $.ajax({
    url: "page.php",
    type: "POST",
    data: formData,
    success: function (msg) {
      alert(msg)
    },
    cache: false,
    contentType: false,
    processData: false
  });
    
});