Change mobile device image ratio to 16:9

[CYON]

Hello,

I am seeking some help with alterations to my shopify project. I am using a theme which I purchased off the internet and I would like to adjust the aspect ratio for mobile device to 16:9. I do not understand code so I will need some assistance. 

I will attach an image for referece. 

When I upload images they are slightly cropped on mobile divices. I would like the whole image to display without this slight crop. I believe it must be done in the code. There is a section as seen in the image, which is for customer CSS so I wonder if this is the answer? 

https://huumyoga.com/

[BSSCommerce-TA]

@CYON, can you give me password to access your page.

[CYON]

Hey @BSSCommerce-TA , I have taken the password off so you are able to view it now. Cheers 

[BSSCommerce-TA]

@CYON, do you mean: 

[CYON]

I just made some adjustments earlier, but i mean the top image "your best self". notice how the image is cropped. I would like to change it to a 16:9 aspect ratio, to fit the image nicely . cheers