How do we know that 'pictures' is in the plural, from the question? Surely 'picha nzuri' can be either singular or plural, and 'a picture' should have been accepted?
I think it's implied since you're talking about several books
Yes, you’re right. Thanks. I should have taken the context into account.
But still 'picture' is a correct translation.
Imaging a school class where every child has his/her own copy of a particular book, that just has one picture in it. Then the use of the singular would be correct :)