Question 1 is the simplest way to ask someone to tell you what's on the wall. Question 3 is also correct if you are referring to a specific spot or object on the wall. (correction: What's on the wall there?) If you want to use "are" then you would have to add a plural noun to the question: What pictures are on the wall? What pictures are there on the wall? Remember: "is" is used when the subject is singular: the WALL is white. "are" is used when the subject is plural: the WALLS are white.

2, 4 and 5 are correct. 1 and 3 are incorrect because picture is plural and the verb "to be," must agree with the thing to which it refers. So it makes not more sense to say "What is on the wall?" "There are some pictures," than it would to say "Pictures is on the wall." The questions for 1 and 3 would work perfectly well with the answer to number 5.