Every few days my login screen image was changing automatically with the new image. But from recent many days it is not changing with the new image. Is it from Microsoft side or is there any problem within my Windows 10?
Your Login images used to change automatically since the windows has a series of images set to automatically change at a given frequency. In your case, you only applied one image thus the login screen has only one image to play making it use the available image in settings. Go to the settings >> Personalization >> Lock screen under background choose picture or slideshow to use your own images or stick with Windows Spotlight wallpaper as shown below. You can set any amount of images as your wallpapers and also set the frequency based on your preferences.