What do I have to do to hit Death with a pumpkin

I believe they unlock still frames of art, which you can access in a particular location in the mission select/hub area.

You will need to use the Diffindo spell to work with anything that is red. You will gain this spell in your first year, after you have been to see Professor Flitwick for the first time, and completed his Charms lesson. This lesson is played after the third level of the year 5.

Diffindo can be used to cut shapes out of red walls, which you can then build into other items. To cut a shape out of a wall, make sure you have Diffindo selected on your spell wheel, then target the red wall to cast at it. You must then stand on the small glowing orange spot on the floor in front of the wall. When you are in place, an X will appear showing that you need to press X. You can now draw the outline of the shape on the wall to cut around it and release the bricks. These bricks can then be built into other objects that can help you along the way.

There are other objects around the game that are smaller than the walls but are still red. You will also need Diffindo to destroy any of these.