Resize Image on Mobile Only

Site URL: https://panda-cylinder-3za4.squarespace.com/

Hi,

I want to resize my mobile image on this page https://panda-cylinder-3za4.squarespace.com/about-salon-purpose: 

I've tried codes I have used before however it's not resizing on mobile (would like it 50% of the size). 

Thanks,

Lee

Add to Home > Design > Custom CSS

/* About salon resize image */
@media screen and (max-width:640px) {
div#block-yui_3_17_2_1_1601099246769_12784 {
    width: 70% !important;
    margin-left: auto !important;
    margin-right: auto !important;
}
}