"Illegal" to recycle pictures? No.
Foolish to recycle pictures when selling a pre-owned bag? Absolutely!
Many sellers of NWT items buy multiples and use the same pictures to sell more than one of the same it. There's nothing wrong with that and ebay doesn't disallow it.
Ebay also doesn't disallow sellers from reusing pictures if they happen to have more than one of the same preowned/used item although for a seller to do that, they are risking a SNAD dispute.
As we see so frequently, especially on the ebay forum, buyers don't read descriptions or if they do read them, they skim then and don't actually take note of all the "condition" text.
Although all the pictures and both bags may belong to the same seller, if the bag pictured is less worn than the actual item that the buyer will receive, the buyer isn't going to be happy and will open a dispute, ding the seller's feedback and consequently, ebay will put a "defect" on the seller's account.
IMO, it's a good idea to photograph ALL used items individually and to show pictures of the actual item in the listings.