To turn on this feature, go to Settings > Personalization > Lockscreen. Note This feature might not be available to all users because it will roll out gradually.
Business Insider: What is a PNG file? How to open or convert the popular graphic file format
PNG, short for Portable Network Graphics, is a popular and high-quality graphic file format. The PNG format is both lossless and supports transparency, making it great for webpages. You can view PNG ...
What is a PNG file? How to open or convert the popular graphic file format