Counter Argument on the images (without seeing any contract drawn)
Most of the time you pay for an image to be made which is exclusive to you, and within the transaction it becomes the Purchasers image to do what they like with. This is because you have paid someone to make something for you.
You do not pay a decorator to come paint your front room and he then specifies you can not hang any pictures on the wall as that will be altering his work......
On the other side some artist work for free but state they get a percentage of money made on the sale of the product which in that case the Artist can by all means claim all copyright and enforce no editing unless a buy license is purchased.
Some Artist use what is called home Stock/ fill stock Ie website designers that own images self created or purchased etc which they will pre fill a poster/ project/ website with as place markers for content or just to show the site populated, you do not normally have the rights to use these images long term. This is similar to lorem epsum text.
That said as I have always said we are not the internet police all responsibility falls on the OP and they are only asking on how to do it not for it to be done, we can teach techniques even illegal 1s and put it as for learning purpose only.
on the opposite side to that argument how would you feel if that was your income someone approached you asked for a specific item and then changed their mind the artist has every right to say hey thats not what you asked for I can do it but I will need to charge you extra.
but on the other side most popular decent digital artists do save layers so it would be 2 mins just to switch off the colour layers and should easily supply them. Of course if it is hand drawn then it would need to be redone so rightfully so charge again.
But should be easy to trace either way and go over with a thicker fineliner.
