I’m confused. You asked for the grant, and you received the grant, not the artist, to produce the posters. You hired and paid the artist for these posters. Since this was money from a grant you should have had the artist transfer those rights to the person (You) / company(who gave you the grant) who provided the payment.
Due to this lack of foresight now VARA comes into play and what a FUBAR that can be.
Though if you want to through in conspiracy, how fortuitous that now 5 to 6 posters exist paid for by a Zcash grant than one can simply pay the artist a fee to modify their work.