Help! Squarespace Form CSS

[Christinee]

Hello,

Im quite the noob so I need your assistance. 

Im building a website using squarespace and im trying to modify the checkbox section of the form through css. I would like every checkbox to have a different image and the items to be in 3 columns rather than one. So far i have managed to edit the size of the checkboxes and insert the same image in all of them (see attachment) but i cannot figure out how to individually edit every single checkbox differently. Please help.

Thankyou!

C