It should be "A picture that cannot be drawn" as the most correct response. "A picture which cannot be drawn" would be commonly understood to mean the same thing but it reads that the fact that the picture can't be drawn isn't an integral part of the sentence and you could leave it out without changing the meaning of the phrase/sentence. For purposes of this course, both should probably be accepted.
Hehe, a good reminder of the differences between which and that. Not many people know this in English.
Introducing more new concepts - such as actions as the objects. I hope they add in some more guides/help soon.
Adding -ㄹ수 있다/없다 to the verb stem describes ability/inability to perform said verb.