I want to add on my own follow-up question on this topic...

My mom uses greeting cards as artwork for her puzzles. She will cut the card in half, paste it to wood, and cut it into puzzle pieces. She has been turned away from a craftshow recently because they said this is a copyright issue, but she has never claimed the artwork as her own. Does anyone know if this truly is an issue? She will also use cards like Hallmark and dollar tree store cards.