Translation:Who painted the picture of her aunt?
Is there a difference in meaning between "bildo" and "pentraĵo" besides simply "picture" vs. "painting"? Also, how would you say "portrait"?
A photograph (foto or fotografaĵo) is an example of a bildo that is not a pentraĵo. Bildo can be any kind of image. Pentraĵo is specifically a painting.
According to Glosbe and ReVo, portrait is "portreto".
Wouldn't this be correct? ¨Who painted her aunt's picture?¨ It was marked wrong.
That sentence would change meaning depending on the context. If you were standing in front of a picture of her aunt then it would mean the same as "kiu pentris la bildon de ŝia onklino" but if you were standing in front of a picture that belonged to her aunt then it would still make sense.