Is repeatedly saying, "May your form be cursed" considered an injustice on the part of the speaker, given their initial ignorance of its prohibition or that it is a supplication?

The statement "God curse your image" (يعلن شكلك) is an insult and an aggression. Ignorance of the details does not absolve one of responsibility. The duty is to repent to God for it, by feeling remorse and resolving not to repeat it. As for the rights of those who were cursed: if you remember someone and asking for their forgiveness would not cause harm, then seek their pardon. If you have forgotten someone or fear that seeking their forgiveness would cause harm, then supplicate and seek forgiveness for them, and humbly implore God to make them pleased with you.