You see the same image because the image file is still the same and the browser thinks it's still the same image. Of course this doesn't happen forever. It depends on the browser and could take up to a few minutes for it to recognize the changed image, through the image's headers sent...

Anyway, since this was a problem high-traffic sites dealt with, we added this new option in the K2 parameters (in v2.5.0), which attached the item modified date to the name of the image file, thus forcing the browser to fetch the changed image.

In the backend, open the K2 parameters, switch to the "Images" tab, set "Force 'item modified date' as URL timestamp on image paths" to "Yes". Save.

Now you won't see this issue again.