It is such a difficult issue to deal with after the fact. I have started specifying a cost per additional image should they want more than they initially purchased. That at least gives a starting point.
Having said that, even if you specify it in an email or a contract, the client relationship may suffer when they are asked to pay for additional elements. It makes is really uncomfortable, so it is extremely important to have a contract in place and to make sure they know what is theirs and what is not theirs.
It is also a great time to ty and retain your copyright over the images.
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