It does sort of sound like they have a fixed budget and are trying to wiggle out of a mistake they made without having to pay additional money they had not planned for. It seems to me you have delivered on the contract already, per their very spesific spesifications. You might suggest they need to provide a new contract, or contract addendum, for the additional art services (assuming they have not bought your soul with the original contract- i.e. unending revisions until they are happy). I'm sure they will take your additional free work effort if you give it to them, but they will probably recognize what they are asking of you if you point out the issue in a professional way.