I don’t understand this sentence, so it’s not grammatically correct.
What you want to say, it’s a long sentence with two things.
First is the portraits you found is painted by a artist.
Second is the artist is dead.
It’s a little difficult to tell this two things in one sentence in Chinese.
I could try to say:
你找到的那些画的作者死了。
Although 画家 is more correct than 作者 here,
It’s kind of weird in pronunciation in Chinese,
你找到的那些画的画家已经死了,this would confused me,“那些画的作者” is better than “那些画的画家”。