If the images are on a Pi within your own network then you need to ensure that they can be resolved from outside of your network. That normally means using a DDNS servicea and updater, and setting-up appropriate port forwarding rules on your router.
I’m guessing that the images appear on your phone when it’s connected to your local WiFi, but that if you turn-off WiFi on your phone and use GPRS data instead then the images won’t be displayed?
A simple solution is to place the images on a public webserver, or a GitHub page, or here…
Pete.