How to tell if an image was updated
You can use ETags - when the app receives an image, there will be an HTTP header that looks like:

ETag: "686897696a7c876b7e"


Whenever the app wants to check for a new image, perform a GET request with this header:

If-None-Match: "686897696a7c876b7e"


If the file is unchanged, your web server will respond with HTTP 304 Not Modified status.