Using a default image for items that have no pictures assigned to them is a common practice that is used by many. Setting it up requires changing specific field mappings and adding the image that will serve as default to the image directory recognized by NSc Sync. It is thoroughly described in KB article #469.
Since the image in question is not actively assigned to any product, NSc Sync might have trouble recognizing it and therefore including it in the synchronization which could lead to lack of visible change. The situation is more probable when trying to overwrite an already existing 'default.gif' file, this is because of cache memory.
To make sure the old image is replaced properly, the new version of 'default.gif' should be assigned in NSc PAM to one of the items without a picture. That way it will be recognized by NSc Sync and consequently applied to all items without an image.
Should the need arise, an actual image can be later on assigned instead of 'default.gif' to the item we used without affecting the picture replacement.