I pictured him as a serious person, but he turned out to be fun.

我想象中他是个很严肃的人，结果发现他其实很有趣。

上句中的picture是动词，表示想象，把某人想象成某某样子就用as。

I am picturing shaking hands with our country's president.

我在想象和国家主席握手的情景。

I often picture myself living in Beijing.

我经常想象自己是生活在北京的。