PeterzAdvertising

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.