Move points along Z randomly !

Hi everyone, i'm very new on the forum and on grasshopper !

It is for an exercice in school i want to create a cloud point who represente values of pixels of an image in a box. Goal is to do a little animation after.

Here is my probleme: when i select the point on the surface GH select only columns and not point (1st image). If I do "flattern" with my point (2nd image) i have the result that i want but whithout the volume of the point: they are all the same, cause i think gh "erase" these parameters with 'flattern".

Do you have a solution to combine those things ? I try everything with lists, trees but nothing works !

Thx if you help me and sorry for my english ! :x