Good idea testing it by changing the save image. The core code uses the same css style and image for the save button in all apps. Maybe your NPD instance is pointing to a different images and/or css directory than your GSM instance. Can you check IIS and see if both apps are pointing to the same location for images and css as your GSM instance?
I guess it could be a browser cache issue, you need to do a force refresh on your client. I'm sure you did that for it to appear in GSM though and tried the same thing for NPD.