I'm no expert but firstly I'd identify the #block-yui ID number for that image and simply set a media query.
Install squarespace block identifier onto your computer.
Then enter your custom code.
Something like:
This is my code to set a custom width on a computer screen
#block-yui_3_17_2_1_1573147737484_74837
{
width: 150px
}
This is my code to set a custom size for the image on a mobile screen
@media only screen and (max-width: 600px) {#block-yui_3_17_2_1_1573147737484_74837{ width: 50px}}
Hope this helps.
I've only been coding for a few months but seems to work for me.
