Answer: A portrait was being painted by him.
Explanation: Generally, while changing from active voice to passive voice or vice versa, we follow some rules.
The given sentence is in Past Continuous Tense. We must rearrange the words such that the meaning of the sentence is not altered or changed.
The sentence is in the following form:
Subject + Verb + Object
To convert to passive we must change the form to:
Object + Verb + Subject
We can also say that after conversion the object will become the subject of the new sentence with a little change in its verb.
So your answer should be:
A portrait was being painted by him.