Image too small on mobile view

Hi! I created a test page in my unlinked section so that I could figure out this formatting before messing up my regular pages. But I am having issues with my image looking way too small on the mobile view. I used this code below, but it's still really small (see the screenshot attached). 

@media only screen and (max-width:640px) { #block-yui_3_17_2_1_1679509101538_2761.sqs-block-image {width:80%; margin:auto}}

It's a book cover that I would like to be much bigger, thank you in advance for your help!

Hi,

What is site url? We can check easier

www.friendshipwithintention.com you can see on the "my books" page how small the image looks. Thank you!

@tunaphan were you able to see it?