Why can't I hide images on Mobile only using the CSS I'm adding?

Hello, Can someone please help me, I am trying to hide certain images on the mobile version of my website using the code: 

@media screen and (max-width: 640px) {
#block-yui_3_17_2_1_1689763110049_14549 {
display: none !important;
}
}

But it's not working! My website is karlmackieshop.com 

Grateful for some advice and thanks in advance.

@graphicabyss This id seems to target a button.

Also code doesn't seem to apply although it's correct for hiding. 

Ah, thanks for replying to this, really cool.

I had pasted in the incorrect ID, this one is correct:
 

@media screen and (max-width: 640px) {
#block-yui_3_17_2_1_1689763110049_14549 {
display: none !important;
}
}

Are you adding to Custom CSS? It maybe an error before so code isn't processed. Because it works if applied directly to the site. 

Hi, yes I am adding it to the top line of custom css at the very top, what does 'an error before' mean?

Many thanks

It worked! Thank you so much! I increased to 800 to hide on iPad as well but thats been amazing to solve this!

@graphicabyss I'm glad that worked! Happy designing 😊