Origin of the drawing (which can be found on Pinterest by searching 'Shitadori') is the artist wondering if Sukuna's fingers get digested and excreted. The topic escalated to a drawing of Itadori Yuuji sitting on the toilet, with Sukuna on his hand telling him to hurry up.

Exact words:
Sukuna: "bro, why is this taking you so long????"
Itadori: "literally shut up, I'm trying to concentrate"

Looking at the drawing can also cause a sudden feeling of motivation which is vital for passing an exam.