Google Cloud Storage: how to update cached versions of files?

I have uploaded thousands of files. However, when a user rotates an image in my website I internally update the uploaded version (re-upload the rotated version via PHP to the bucket). But, google keeps displaying the same image again and again.

However, if I append ?r=[number]

Where [number] is some random number, the updated image is displayed correctly. And if I then remove the ?r=[number] part from the URL the old image is displayed again.

Is there a way to tell google to update the displayed version?