Detecting change in profile or background image


The app “Smarsh Archiving” periodically retrieves ‘profile_image_url’, ‘profile_background_image_url’ and if the image changed from the last time retrieves the new image and stores them for display. Between Jan 21, 2014 & Jan 23, 2014 the app received:
“profile_background_image_url”: “”,
"profile_background_image_url": “”,
for alternating requests for the account id “209393185”. This happened for a few other accounts, but stopped happening since Jan 23,2014.

What is the best way to detect a image change in the profile?