Hide a banner image + center an image exclusively on mobile screen

Site URL: http://preciose.squarespace.com/accueil

Hello,

I'd like to hide the banner on my homepage just on the mobile screen because it's not good looking (see file attached)
I've tried some code but it's not working...

Also, I need to align the symbol in the middle of the page just on the mobile screen but it's not working... (see file attached)

Can you help me, please? :)

What is access password?