It is said that if one dies the picture of hell for the disbeliever or heaven for the righteous person would be shown before they were move to the grave. Then what is the need of questioning in the grave?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

There is no contradiction between giving glad tidings to the believer to enter paradise, informing the non-Muslim to enter Hellfire when they die and after their death, and the questioning of the two angels in the grave. The questioning of the grave has many benefits among which are:

1)     Getting a confirmation from the believer and the non-Muslim: The reality of the belief of the believer will become obvious by being able to successfully answer the questions, and the disbelief of the non-Muslim would also become apparent as he will not be able to answer the questions successfully.

2)     Honouring the believer and scorning the disbeliever.

3)     Being afflicted: That is why it is called 'The trials and tribulations of the grave', so that the sins of the believer are thereby wiped out, whereas the disbeliever will be punished and in sorrow.

4)     Showing the honour and dignity of the Prophet by being questioned about him, as it is reported in the narration, "Who is this man who was sent to you?" [Abu Daawood]

There are also other benefits.

Allaah knows best.