Conduct research on the art work using at least 4 sources written by experts.
How to write paper:
Your paper should consist of a sound argument (aka, thesis) and you must successfully support it with evidence-based research. This is how research progresses.
An academic art history paper is much closer to math than it is to creative writing: it’s like a puzzle that needs solving. You’re basically stating your opinion BUT your opinion is based on factual research!
It shouldn’t be based on your personal feelings or opinions. An argument also is not something that we already know.
So, “Leonardo da Vinci was an artistic genius” isn’t a great argument because we all know that (and everyone knew it when he was alive, too). A better one would be “Venice’s architecture reflects its broad and lucrative trading networks in northern Europe and with the Middle East.”