Return Values from a Function inside another function Ajax, Rest

Question

i'm struggling to understand why the code below when the pictureRetrieve function called from inside the Completedetails() function it returns undefined. Despite knowing that the values exist especially when i check the console. Checking online it seems the problem maybe as a result of the asynchronous nature of Ajax. it maybe an area i need to improve on but i'll be grateful if someone could spot where the issue is and how i can fix it

function CompleteDetails() {
  var requestUri = _spPageContextInfo.webAbsoluteUrl +
    "/_api/Web/Lists/getByTitle('OrganisationalChart')/items?$select=Id,user/Title,user/Id,user/Name,Boss/Id,Boss/Title,PictureUrl,Designation&$expand=user,Boss";

  $.ajax({
    url: requestUri,
    type: "GET",
    async: false,
    headers: {
      "ACCEPT": "application/json;odata=verbose"
    },
    success: function(data) {

      var dataResults = data.d.results;

      console.log(dataResults);

      for (var i = 0; i < dataResults.length; i++) {
        var userDetails = dataResults[i]['user'].Name.split('|')[1];
        var employeeId = dataResults[i]['user'].Id.toString();
        var employeeName = dataResults[i]['user'].Title
        var Designation = dataResults[i].Designation != null ? dataResults[i].Designation : '';
        var reportingManager = dataResults[i]['Boss']['Id'] != null ? dataResults[i]['Boss']['Id'] : '';
        reportingManager = reportingManager.toString();

        //Get the user profile picture for each user
        userPicture = pictureRetrieve(userDetails);

      }

    },
    failure: function(r) {
      alert(r.d);
    },
    error: function(r) {
      alert(r.d);
    }
  });
}
var pictureRetrieve = function GetPictureUrl(user) {


  var requestUri = _spPageContextInfo.webAbsoluteUrl +
    "/_api/SP.UserProfiles.PeopleManager/GetPropertiesFor(accountName=@v)?@v='" + user + "'";

  $.ajax({

    url: requestUri,
    type: "GET",
    async: false,
    headers: {
      "ACCEPT": "application/json;odata=verbose"
    },
    success: function(data) {


      var DataRes = data.d.PictureUrl;
      var PictureDetails = data.d.PictureUrl != null ? data.d.PictureUrl : 'nouserimageurl';
      console.log(PictureDetails); // i get a value for this everytime it is called;`return PictureDetails;                  


    },
    failure: function(r) {
      alert(r.d);
    },
    error: function(r) {
      alert(r.d);
    }



  });
}