According to the Gmail Blog, there has been a change in the way the Gmail app handles images. You may have noticed that, a lot of the time, an email with images will have the “Images are not displayed” message, with options on how to show them. Now, however, Google has changed how images display, saying that ” 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.”
Google also says that this makes messages “more safe and secure”, as images are now scanned for known malware and viruses.
You can still turn off the option to auto-display images by going to the General tab under Settings in Gmail, and choose “Ask before displaying external images”. The changes apply to desktops (currently rolling out), as well as the mobile app in early 2014.