"Have you ever wondered why Gmail asks you before showing images in emails? We did this to protect you from unknown senders who might try to use images to compromise the security of your computer or mobile device.p.s. you can still default to not displaying images; check the Google post link below for details
But thanks to new improvements in how Gmail handles images, you’ll soon see all images displayed in your messages automatically across desktop, iOS and Android. Instead of serving images directly from their original external host servers, Gmail will now serve all images through Google’s own secure proxy servers."
Official Gmail Blog: Images Now Showing