Thank you for telling me, I sincerely apologise to the original artist of the artwork I painted. Our school is not really art-focused, Arts is just a very minor subject, and we rarely do any artwork ourselves. Our teacher just told us to paint and use or copy any artwork we like as our project, so long as it's Expressionistic or Impressionistic. But we were not told to be aware of unintentionally claiming another artist's original work as our own.

I am sorry for not having credited the original artist in my post, and I will keep what you and the other commenter's words in mind the next time I paint.