SetUserImage.cs

using Microsoft.SharePoint.Client;
using Microsoft.SharePoint.Client.UserProfiles;
using System;
using System.IO;
using System.Security;

namespace ImgSetter
{
    class Program
    {
        static void Main(string[] args)
        {
            using (ClientContext clientContext = new ClientContext("https://yoursite.sharepoint.com/"))
            {
                SecureString passWord = new SecureString();
 
                foreach (char c in "yourpassword".ToCharArray()) passWord.AppendChar(c);
 
                clientContext.Credentials = new SharePointOnlineCredentials("[email protected]", passWord);
                
                var peopleManager = new PeopleManager(clientContext);

                System.IO.Stream stream = new FileStream(@"C:\imgToSetAsProfilePic.jpg",FileMode.Open);
                
                peopleManager.SetMyProfilePicture(stream);

                clientContext.ExecuteQuery();

                Console.WriteLine("Profile Picture Set");
            }
        }
    }
}