I heard they were actually nice people even though I picture them as being quite scary.
I imagined them as scary people even though history tells us they were actually kind and gentle people.
この場合、次のような言い方ができますよ。
ーI heard they were actually nice people even though I picture them as being quite scary.
「彼らは実際は優しい人たちだったと聞きましたが、私は結構怖い人のイメージがあります」
to picture someone as ...「人が…だと想像する」
ーI imagined them as scary people even though history tells us they were actually kind and gentle people.
「歴史は彼らが実際は優しい人だと言うが、私は彼らを怖い人だとイメージする」
to imagine someone as ...「人を…だとイメージする」
ご参考まで！
カナダ 
