This happens because when you go from the Shopping Cart back to the Product it includes the previous settings of the images ...
Having the Reset link is a quick way to "clear" the previous images so additional ones can be loaded ...
To better understand this:
1 add the 5 images to the Shopping Cart
2 go back to the Product via the Shopping cart and just click the Add to Cart ... now you have Qty 2 in the cart ...
Change the quantity to 1 and update ...
Now go back to the Product via the link in the shopping cart and you will see the 5 images ... Manually edit the URL and remove on the products_id everything from the colon ( : ) including the colon and you will see that the images are not listed ... Now just add 2 additional images, different from the first 5, and click Add to Cart and you will see the 7 images, 5 on one Product and 2 on the other ...
Again, this is the idea behind the Reset Product link that I gave you, to provide the customer will an easier method to add additional images ...